Sounds like you have broken something somewhere.
The jittery movement sounds odd - sometimes it happens when you are connected to a server and you have removed a block beneath your feet but the server doesn't think you have (this can be due to lag).
However you being in the middle of an ocean makes that unlikely.
Possibly the standing on the water could be the game thinking you are still inside the boat and when trying to move it snaps you back to the boat location, this would explain the jitter and slow movement.
Whatever the exact cause of the issue however, it should be fixable by restarting the game - just be prepared to swim the moment you reconnect in case you start underwater.
If for some reason after restarting it is still broken then you can try re-entering the boat/leaving it a few times, trying to jump around/move away from the area you are stuck in.
If all else fails then you might have to resort to using commands, the teleport command might work if you teleport yourself using relative coordinates:
/teleport ~4 ~1 ~0
Will teleport yourself four blocks along and one block up for example, and might move you outside the area that seems to be causing issues.