This guide only contains the keys in Ryukyu, not Kamurocho. Locations for those can be found here:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2382815466
They unlock once you reach Chapter 4, but one is locked away until Chapter 10.

There is only one achievement tied to the lockers themselves, and it is Key Collector, which requires you to open 10 lockers in the game. Past this point, there's no real need to open all of the lockers aside from the fact that many contain the OP gambling items, which will help with other achievements. Those with gambling items as rewards have been labelled as such.

There are 50 keys total in Ryukyu. Three of these are found at Morning Glory, and three can be found inside the Public Market area that is found inside the arcade/ Public Market North. As an FYI, Public Market North is the southernmost area on this map, it's the area also known as the arcade in-game, and the cause of much confusion. It is within Public Market North that you enter the double doors to get to Public Market 1st Floor and 2nd Floor. The inside will be referred to by their floors, and the outside area will be called arcade/Public Market North.

1. Head to the arcade/Public Market North (southernmost area on the map) Look for a shop with a green /light green striped awning. Go to the immediate right of it, enter first person, and you'll see it in the grates up above.
   
2. Inside Aqua Sky, on the ground behind the billiards table from where you walk in.
   
3. It's on the floor inside the weapons van.
   
4. Located inside the Public Market, 2nd Floor. This is the area located within the double doors inside the arcade/Public Market North at the bottom of the map. Facing the eating area, look up at the vents in the far left ceiling. The key is on a light fixture, enter first person to get it.
   
5. Find a tree in front of a building with bright red lettering on the front. The key is up in the tree, use first person to get it. GAMBLING ITEM: Bust Amulet.
   
6. Go to the train station and stand underneath it by the river. There is a pillar with a pipe going up its length and a tree in front of it. Enter first person and you'll see the key up on one of the metal bars holding the pipe. It's a bit high up.
   
7. Head to Morning Glory. Facing the house entrance from the beach, head toward the left down the sandy path. The key can be found on one of the fence poles.
   
8. Inside Ryukyu Night Guide, it's on the floor next to the employee.
   
9. In the alleyway, there are planters(?) on the ground by the wall. The key is in between two of them. GAMBLING ITEM: Black Jewel.
   
10. Inside Smile Burger and you'll see the key in an empty chair.
    
11. In the small area underneath the coin lockers, to the right of the stairs.
    
12. Middle of the street, next to the taxi. GAMBLING ITEM: Royal Joker Card.
    
13. Inside Mahjong Camp, on the floor by the intermediate players.
    
14. Inside the Gambling Hall, in the middle of the floor.
    
15. Inside Kamiyama Works, on a shelf by the door. GAMBLING ITEM: Lopsided Cigarette.
    
16. Inside the Shogi place, in between the first and second rows of empty tables on the left.
    
17. This is locked until Chapter 10. The key is found on the floor of the Ryudo Family office.
    
18. In this small alleyway in the arcade/Public Market North, the key is on a red bench.
    
19. Go into the small grassy area off the side of the road underneath the train station. The key is on the ground.
    
20. Center of the floor inside Maeda Drugs.
    
21. Head to the fish seller inside the Public Market, 1st Floor. This is the area located within the double doors inside the arcade/Public Market North at the bottom of the map. Enter first person and look up onto the signs above the shop, the key is situated between two large signs.
    
22. Middle of the road, at the intersection.
    
23. The key is a bit hard to see in the daytime. It's found in the water on the Morning Glory Beach. It's to the left of the entrance, where the water meets the sand.
    
24. On the ground inside the parking lot.
    
25. Inside Aqua Sky again, this one's found to the left of the bar.
    
26. This one is found on the floor right in front of the buildings here, by some trash cans and a bike. GAMBLING ITEM: Blackjack Amulet.
    
27. At the end of the road, on the ground.
    
28. Head to the stairs of the train station. When facing them, take the left road underneath it. There's going to be a building with a pink awning and a red sign hanging off it right next to a building with a large red sign on it. Look in between these buildings in first person, it's a bit high up, on a pipe.
    
29. By the taxi, on the sidewalk.
    
30. Inside M Store, in between the middle two rows, very center of the store.
    
31. Located inside the Public Market, 1st Floor. This is the area located within the double doors inside the arcade/Public Market North at the bottom of the map. Once inside, take a left. The key is on the ground in the middle of an intersection.
    
32. Look for a large pole standing between the Ryukyu Street building and one with a sign saying "Ayako" in red lettering. It's up on the pole, you need to go into first person to see it.
    
33. By the Coin Lockers, there's a lady who lost her key. She's going to ask for you to look for it. It's found underneath the second bridge, on the ceiling, above where you're standing after you're done talking to her. She gives you the key as a reward.
    
34. Facing South Island, look to the left toward a building corner with signs wrapping around it. The key is on top of the highest sign, you need first person to reach it. GAMBLING ITEM: Piss Mountain Stone.
    
35. Look for a parked black car. Look to the left, the key is atop one of the air conditioners. GAMBLING ITEM: 1-2-3 Gum.
    
36. Inside the M Store, it's on the counter right as you walk into the store.
    
37. Look for a store with red, white, and blue lanterns hanging in front of it. The key is on the ground in front of it.
    
38. Right above the Coin Lockers, use first person to get it.
    
39. In the middle of the length underneath the train platform.
    
40. In the back hallway of the Morning Glory Orphanage. Use the center door to enter the house, the one with a long hallway in front of it. Head down it and turn left at the end, and the key's on the boxes.
    
41. On the ground in the southwest corner of the arcade/Public Market North.
    
42. In the small alleyway, look for balconies that hang up top. The one you need is across from a red sign. You need to use first person to search the balcony, the key is on it.
    
43. Head up the stairs that lead to Kamiyama Works, the key is on an AC unit hanging on a wall in the area in front of the shop on the second floor.
    
44. This one is the "haunted locker," a part of the Seven Mysteries of Ryukyu, and the substory takes place at night. You'll see the group of schoolgirls outside the M Store near the Coin Lockers. Talk to them and then go to the lockers, and you'll get the key as the substory progresses.
    
45. On top of the Ryukyu Street bus stop, go into first person to spot it. It's visible on one of the short sides.
    
46. Look for a bunch of repeating blue and orange signs going up in a line on the side of a building next to a shop with red, white, and blue lanterns hanging in the front. Look at the right side of the awning in first person, the key's on the corner.
    
47. Look for a building with a blue "HOTEL" sign in the front, with a small green sign hanging off the side that says "COFFEE." The key is on this green sign, use first person to get it.
    
48. Come close to the railing of the bridge. There are pipes running parallel to the bridge railing, and you'll see the key on it easily when you enter first person.
    
49. Run along the river and head to the second bridge that's on the other end from the key above. Same as before, look over the edge, and you'll see the key on the other side in first person. This time it's to the left, on the rocks.
    
50. There's a small stairwell across the street from the Ryukyu Night Guide, the key's found inside, you need first person to reach it.

Thanks for checking this out, hope this helped someone.

Got any questions? Need any help? Did I mess up somewhere or can I improve something? Feel free to leave a comment here or message me directly.

[EDIT 02/20/21]: Fixed the map, it was missing #41. Thanks for bringing this to my attention, Moe ChiaKi!

Here's an empty HD map, devoid of any symbols, in case anyone wants to use it for anything. It's a composite of in-game screenshots, just like the one above: