Can't get on boat

[Sir Whiskers]

I had similar problems with KB:TL, where I would exit the boat and not be able to reboard. Unfortunately, if moving around a bit didn't fix the problem, my only option was to reload from a previous save.

I pulled up an old save game and tested your situation in the Underground Sea. I tried exiting and reboarding a couple dozen times, and was able to do so every time. I even tried moving the camera angle around to see if that made an impact, but it didn't appear to.

So I suspect this is not a problem unique to the area or exiting onto a platform - you appear to have just gotten unlucky in the exact spot where you exited. You might try moving the camera to an angle directly above the boat, in case that makes it easier to reboard - it didn't make a difference for me, but you never know.