Although at least one geocacher survived swimming out to this island last year, it is not recommended. There are boats that can be rented at the nearby marina, or bring your own canoe, kayak, or inflatable boat for a quick paddle out to this event. Obviously, a bunch of attendees could and should probably go in together on a boat rental to split the cost.
This cache is rated 1/5 for D/T. Difficulty 1 because it will be super-easy to spot the group of people on this little island, and Terrain 5 because it requires special gear (a boat) to get to the posted coordinates.
There are also 2 traditional caches within very short walking distance once you get out to the island.