first, i don't know anything about the new USA Waterski program, i haven't looked into it.
personal liability probably won't help you out if something happened while you were giving lessons. because at that point, it's not personal- it's commercial. nintzel has pretty good advice- but a state farm policy does not have to protect you if someone is paying you for a service and there is an accident related to that service. that "through the roof" number nintzel is talking about, if you're wondering, could be close to $5000.
that belter insurance agency that liquidforcefan mentioned looks pretty good. you could get a local independent agency (not Statefarm or Farm Bureau) to do the same thing- only the local agent may not have experience with this category of insurance so your call on that.
my advice is to not take people out that would sue you! and get a good waiver, and still get the commercial insurance!
bottom line: with the costs involved, it's hard to give lessons part time without losing money or risking personal assets.