Lake Ownership is not nearly as glamorous as it sounds. Be prepared for HUGE amounts of work and less ski time. I was able to ski WAY more before I owned a lake, and that was with having to fight the river taking slalom courses and jumps over the dam every year. Never would I ever even consider having more than 1 partner in ownership or lease of a ski lake. Multiple partners never work out, and many times a single partner doesn't work out. If you let people ski, don't charge them a dime. It is simply amazing how much help you get from someone who wants to ski at your lake when they aren't charged anything. Once money is exchanged, not only does your legal liability change, but your member's expectations change. If you do charge them, be prepared to be expected to fix everything from a buoy floating to high to making sure the port o let has toilet paper. Don't ever expect that you can have a club that rents a lake and you are going to share the responsibilities evenly, it never works. Someone HAS to be in charge, and that person has to be prepared to be the arsehole.
For insurance, since USAWS sold out it's members for the interest of the insurance company, I no longer make the members join USAWS, nor do I join as a club since there is no longer any benefit for me. I make all members name me as additional insured on their boat owner's policy and set a high minimum liability limit. I also carry an umbrella policy. As far as rules, make them short and simple and expect to have to remind the members of the rules from time to time. Someone has to be the bad guy.
My rules are that each membership gets you one spot in the rotation. If you are a single member, you get to ski every time your rotation comes up. If you are a family of four, one of the four gets to ski when their rotation comes up. A rotation consists of 6 passes of slalom or tricks or 3 jumps. Keep membership small enough so that time is not an issue - EVEN if this means you don't have enough members to cover your costs of running the place. I have a rule that I run my lake by: I will keep membership low enough that every member WILL get to ski at least two sets in the evening after work, and as much as they want on the weekends. IF for some reason they lollygag in between skiers or boats, it will be their own fault they don't get to ski two sets in an evening. It will never be because there are too many people waiting to ski.
Bottom line: For me, I would MUCH rather pay $4200 a year to someone else for a membership to their lake than to own my own and have 7 members at $600 each. Believe it or not, I would come out ahead in the end dollarwise and timewise.