This date allows everyone (including us) a bit more prep time than our previous July 4th weekend proposal, doesn't interfere with holiday plans, is a year from seasteading08, is a good time in relation to Burning Man (long enough for people to have recovered, short enough for them to still have some post-festival glow), and is during the full moon.
The river delta location has a number of advantages, including that it is already used frequently for (smaller) boating parties, so our use will be uncontroversial. It is not a challenging environment (beyond being on water) as there are few waves, but we think that's fine for the first Ephemerisle. We will likely feature a big wave contest during the festival where we drag potential seastead models out to the bay and maybe even the ocean. The hope is for the big wavecontest entrants of 2009 to become the standard platforms/vessels of 2010, which would be in a higher-wave environment, and would feature a contest for models to push the boundaries yet again, so we can incrementally make our way to an Ephemerisle in international waters over the course of 3-5 years.