I would agree with HyLlyte.
The nut of it all will be an insurance policy that covers both of you. As the equity owner many places will go after you in the event anything goes bad of any nature. They won't even care about the partner. That means after they get done with you then you have to go after your partner. That won't go well or easy. You pay twice.
The process of assigning liability to the non equity partner is the legal hassle you'll need a lot of expert legal advice on. Internet
legal advice won't make it for this agreement. The partner really faces the easy part of this deal since they have almost no downside. You however gain double the liability in exchange for a fixed amount of money
When the equity is equal you can usually count on the two partners having a similar interest in the boat and while disagreements come up you both are on even ground and will both act in your mutual interest because you really are forced to. This deal does not even being to start like that. Relationships don't usually get better when they don't start out even.