Ok, so the quickest way to get this to work is like this:
1: shrink that infantry pathbox to the size of the platform, you dont *strictly speaking* need it to be any bigger for now. Give it a Cell size of about 0.1 and character radius of 0.4, so that the bots will be able to navigate around any future obstacles you may place without issue.

2: have the shore flag's protect radius extend to the shoreline. This will tell the boat bots where to land when attacking that flag. Be careful with some of your extra-large protect ranges, this may cause the bots to beach themselves in some unintended locations.

Now, that is the bare minimum you need to make bots land the boats on the beach. You dont even really need that boat pathbox, since boats already can sail on any body of water. BUT. If bots get shot off a boat before they get to the terrain limit of the map, im pretty sure they will be stranded. You may want to place a giant shape primitive underwater with an infantry pathbox over it (you can leave it's settings default this time). Use at least one path link to link the end of the box to the very edge of the underwater terrain. This will allow boatless bots the freedom of swimming to shore.

And while i said that you dont *need* that bid boat pathbox earlier, i also noticed some bots taking some wacky wide routes to the beach when i removed it, so limiting boats to that narrow corridor may be for the best. Remember, pathboxes create navigable zones over solid terrain/objects (except for boat boxes, which do so over water), but boats cannot enter or exit a boat box without a path link permitting them to do so. The same applies for infantry exiting/entering infantry boxes. But a boat can cross over an infantry box no problem, and vice versa, which is why the first above solution works without links.

I also noticed that APCs dont get used by bots... not sure why that is, but it may be that they use a car or infantry navmesh to get around. So you may need to try placing that shape primitive underwater with a pathbox over it to make the APCs work, but im not 100% sure...

Anyways, like i said, at the bare minimum, just do the first 2 things to get boats to work. It will be messy, but functional.

Also, this has nothing to do with pathfinding, but the 900 palm trees you placed really lags the map out during play. The editor is kinda bad at dealing with so many trees, especially palms, so if you want this map to be playable on weaker/average computers, consider reducing the palm tree count to around 150 ish.

Hope this helps.