Well I’ll assume the vessel is owned by someone else and you are chartering it for a party. If this is the case then all you would need is an event liability policy, which will cover bodily injury and property damage which you as the party host are liable for from something that happens during the event. The thing is you’ll definitely want a high limit (at least $5 million) because if something serious happens that sinks the vessel, and you’re liable because of something the party attenders did (unlikely but possible) then you on the hook for a very expensive vessel plus ensuing injuries (considering most 65 foot party vessels can hold 200 people, that’s a big exposure). If alcohol is being served then you be looking around $500 for a one day policy, but this can vary depending on the number of guests and where you are located.