Ephemerisle is almost here! You probably already have most of the gear you need (tent, sleeping bag, etc), but there's some gear particular to the marine environment that might want to purchase before you go. In making this list, I tried to list the least expensive item that I thought had adequate quality.
1. A personal VHF/marine radio. With these radios, you can communicate with other boats, ferry drivers, and emergency responders. They will save you a lot of time and frustration. Note that they need to be programmed, and there's a learning curve to operating them. It's a good idea to practice using them before you need them.
It's best to buy a proper marine radio like the Uniden Atlantis:
While less expensive, non marine radios, like the Baofeng UV-9R radio will work, relative to marine radios, a) they're not certified for marine operation (and therefore, illegal to use on marine frequencies) b) don't float c) are more difficult to use/program.
2. A water resistant headlamp. You will need your hands free to pull anchors, tie ropes, and operate tools. It also increases your ability to see where you are going, and prevent falls/scrapes, etc. Note this lamp takes 18650 batteries.
3. Life jacket. It only takes 5 minutes to drown or suffer serious brain injury if you fall in the water. A life jacket will help you stay alive until someone can find you.
4. Personal lighting. It's very dark on the Delta. Personal lighting will help others to see you if you fall in the water.
5. Whistle. With music and power tools running, it can be hard to get people's attention in an emergency. A whistle helps cut through the din.
6. Dinghy. As bikes are to Burning Man, so small watercraft (kayak/canoe/rowboat) are to Ephemerisle. There are less expensive kayaks available, but I prefer the dinghies, as they have more carrying capacity, and are more forgiving of exit/entry skill.
Note that all inexpensive inflatables have high "windage". This means that they are have a lot of surface area exposed to the wind, and can become very difficult to move against the wind. In years past, several people have had to be rescued because they were pushed to shore by wind and current.