This achievement can be very hard to get. Especially if you do your first playthrough on hard and rarely moving from island to island. At some point, you realize that you don't have this achievement. But you want it because reasons.
You read guides and discussions. Because if you going to grind you'd better do it smart, right? You learn about Alt+F4 tactic and then your nightmare begins.
Hours and hours of grind and nothing happens. Your friends block and report you, because of annoying "Your friend now playing ..." Steam pop-up windows.
This ends now.

For some reason developers decided that "old man" event needs additional conditions for its appearance.
This event will NEVER happen until you experience two (2) merchant ship events. No matter will it be guard battle or simple robbery.
That's it.
So if you do "begin from the start farming" this would be your course of actions:

1. Start game, play till you get your ship;
   
2. Navigate the sea and Alt+F4 untill your get "Sails on the horison" -> "Merchant ship" event;
   
3. "Head toward it" and finish event;
   
4. Go to land, win a battle;
   
5. Navigate the sea, get any event, finish it (events of same archetype never happens in a row);
   
6. Go to land, win a battle;
   
7. Do steps 2-6 one more time;
   
8. Navigate the sea and Alt+F4 until your get "Sails on the horison" -> "It's an old boat" event;
   
9. Buy an item from old man by 400 gold (you have enough gold in possession, aren't you?)
   
10. ...
    
11. Celebrate!

Some information on game mechanics:

• Sea events only occur after successful battle, except dungeons battle;
  
• Two same archetype events never occur in a row (archetypes: Bottle with message, Ship, Man overboard, Treasure chest)
  
• You can Alt+F4 until last event's pop-up window and event will not count. Means after restart you still will expierence events in the sea;
  
• Besides "Alt+F4" (as I was informed by friendly community member) you can also just exit to main menu and reload the game. Didn't test this way myself though.

Best game in series.
For me all good impressions was weared down after FOUR weeks of grind. I almost lost sense of reality. Eventually I had to download special tool and go look at the source code of the game. So yeah, that desperate.
For devs, i say this: "Зачем так сложно?". They'll understand.
For brothers-players: "Good luck and happy achievement hunting!"


P.S.
My first guide, so feel free to criticize =)

Fixed steps order in "solution".
Thanks to Malone86 for finding the problem and for his patience till I understood what it is.

Also thanks to Omnifighter for pointing out that "Alt+F4" is not the only way to re-roll the sea event.