I would keep the group small (2-3 max) and pretend like everyone is married to everyone else. For example, use your nice words, be considerate, don't be a jerk etc. Written words/contracts are great but at the end of the day you don't want to rely on them. In a disagreement with small groups of friends/skiers you need to fall back on personal relationships to resolve issues.
I would also make sure that everyone in the group has the same mindset about how nice you'all want to keep the boat cosmetically and how the boat is maintained. Example - someone who never wipes down their boat would never be in my group.
And make sure insurance is in everyone's name and clearly define who drives the boat.
I hope this helps.