You'll need a place to sleep
The most common option is to rent a houseboat. The main houseboat-renting marina in the area is Paradise Point Marina. A typical houseboat sleeps 10. With the shoulder season and "repeat customer" stacking discounts, a 4-day Summit houseboat rental comes to $1050. This it not bad at all split 10 ways, when you consider that each person gets 3 nights of lodging for slightly over $100. At this point, all you have to spend to get the boat is a $300 deposit. The balance is due 45 days prior to the event. To book, ask for Vicki at Seven Crown. The additional costs of fuel, oil, and insurance will run around $200.
Paradise Point Marina also rents patio boats, and grants special permission for the event to keep them out overnight. Four to six people can comfortably sleep on the floor of a patio boat in sleeping bags. Patio boats come with an ice chest, but do not include a bathroom, water tank, or grill.
You do not need a boating license to operate a houseboat.
 Buy a small boat
This is less ridiculous than it sounds. Last year one attendee bought a small speedboat and trailer for $750. I think he resold it afterwards. As a nice benefit, he was able to help the community by ferrying lots of passengers to H&H marina.
 Buy a large boat
Buy a large boat and sail or motor up to the event. There are navigatable waterways all of the way from SF to the event. Or... convince someone who owns a boat that they want to attend the event, and join them as crew. Lots of people with boats at local marinas could be itching for a voyage.
 Build your own craft
We've seen several projects of this sort. They run the gamut from small craft that can be put together in a weekend to large, ambitious platforms. If you're undertaking a project, you should consult the mailing list for advice. Learn from others' successes and failures!
 Attach to the community platform
The team working on the Community Platform is looking at modular designs for stable sleeping platforms that can be added like lego blocks.
 Sleep in an inflatable boat
It's been done. It's risky. Your boat may pop. Your stuff may get wet.
 Buy a big old boat, spend months refurbishing it into a giant crazy epic circus show boat, and then fill it with all your friends
Perhaps the least likely option of all. But when there's a Will, there's a way.
 Be a day tripper
OK, you can be lame and just show up Saturday for a day. Check with the various islands to see what's required to visit.
 You'll need to get from the marina to the event
If you're in a powered boat, this is easy.
If you're not in a powered boat: H&H Marina is the closest to the event, but the distance involved (roughly 3 miles) is still long enough that you won't want to paddle. Thus, if you've built your own unpowered floating craft, you will have to arrange with one of the houseboat owners or speedboat owners to get it towed to the event.
If you're joining a houseboat: We will self-organize regular shuttles from H&H to the event like we did last year. Join the facebook page and ping Chris Rasch and Terry Floyd for info on how to get to the event site from H&H Marina, or check the 2012 transportation document.