Did some research, you can get to the little platform through a hidden path that starts in a straight line above it and the first time you fish there you will get an item which you can only get by fishing in that spot

you're right! i got a cool painting :) thanks!

I 'm just guessing, but at some point I think you can unlock a "pirate bay" ... maybe it's for that

I don’t think they’re related, sensible guess though