Simply come up with a price for your partners half. As low and as realistic as you both can agree to.
Document your entire relationship as best as you can. Buy him out. Then never get back into any kind of boat or other financial relationship with him again. Not because he went BK, because the court will watch you and if you start boating a lot together you will lose your boat and your money. The court will see that as some sort of shenanigans.
Do not stress to hard. Remember, it is totally legal for you to be partners in a boat. It is also everyones right to protect there equity and or investments.
Buying his half out is really your only option. If you let the boat become part of a BK you will have to get in line with everyone else and likely get pennies.
Keep it clean and simple.