One crucial point of info thats missing, is the tenth if a volt thats to the right of the decimal. Why is this so important? Because 12.0V is a 75% depleted battery and 12.8V is 100%. 

I have seen these work but only when one member of the partnership acted as the Owner (and maintainer) and the other partner acted more like a passive investor and occasional user.  that can work but it's got to be the right blend of personalities and styles. 

I would put a clause in the contract that allows a partner to sell their share to the other partner at X% discount under FMV of the underlying asset in the case of unilateral termination of the contract.   You shouldn't have to take a bath just because the other guy is willing to if the market dips and he all of a sudden wants out of the investment.  screw that entirely.  

I did this three years ago with a really good friend.  10 months later he wanted out and now I own the boat.  Be prepared.

Got it - thanks for the help!

I don't know if there is any way to talk you out of it, but there is now way I would do a "partnership" on a boat.

There is a reason this type of thing is know for ruining friendships and putting people in desperate financial positions. You will almost certainly be the rule, not the exception, and I hate to see that happen to anyone.