Could you please post a complete list of your enabled mods?
I installed [aircraft realism] and [vehicle physics for 1.1+] and the small boat behaves normally.
The 'max speed' of the small boat is 58km/h for me (the max-speed that is visible in the UI), the actual speed is between 20-30. I doubt any vehicle can go faster than than the max-speed as defined by the game.