Well grappling is a very sketchy part of the game, some have done it, others not.
Some navy ships end up not sinking, after youve got the red hull bar down, if u come along side when this happens you can see circles, u can grapple then and attatch hooks.
However even after that i havent manged to board a ship, it just ends up staying connected to my ship until i leave my ship. LOL.
The game hasn't yet made boarding the ship a requirement for a task that u need to board for to complete, seems u can do these tasks by defeating the ship required.
Being that i haven't actually boarded a ship i dont know what u can find there, altho only ever seen some silly guards on there, can fight them on land easy enough.
So if it was me, i wouldnt bother with it, it seems a buggy area of game anyways.
'Keep it Kool
'