Partner B takes the shared boat out, gets drunk, and runs someone over. Liability insurance isn't going to cover an accident that happens in the commission of a crime (boating under the influence) The other party is going to name every person on that title as a party to the lawsuit.
Hard NO!!! Nothing good will result from it.
The only way to do it would be set up a LLC that all owners pay into for shared maintenance. Basically rent the boat to yourselves. Plane ownership shares are often set up exactly like this, so it isn't unprecedented.