The event was conceived of in 2001 by Patri Friedman as an incremental path towards self-governing seasteads, here is the original & highly dated website. The first event was organized in 2009 by the Seasteading Institute (http://seasteading.org/)) to promote the cultural concept of seasteading - the construction of autonomous floating nation-states - in a hands-on and accessible way. TSI bravely did this event with no insurance the first year.
In the second year, TSI decided not to sponsor the event about a month before it was to begin because the insurance costs were too high ($500 - $1000/person). As a community we decided to show up anyway and create an unofficial self-organized event. Despite the lack of central organization, it still worked quite well, and around 120 people showed up and had a great time.
For 2011, there were no central organizer, but things came together great for another Not-Ephemerisle Event.
After the 2011 event, TSI officially handed over the Ephemerisle name to the community, and the event has continued gloriously ever since.