It all depends what you like, what boat you go on etc.
There are the short (3hr) party boats, lots of drinking, partying, dancing. The issue for me with these is that they start quite early (4pm), so if you planning to go out till 8am its a very long day.
There are also the all day sailing catamarans, these are more relaxed and usually include food, drink and watersports. For me these are better value, and more enjoyable sober.
What kind of boat party do you have in mind?