Cast your net far and wide, Steve.
I've been organising pool runs since 2008, and the majority of those who attend aren't members of any particular model submarine group. Bear that in mind. Oh and I recommend doing a flyer for the event, it sounds like the facilities will come free or cheap, so set a date and press ahead. Don't wait for people to folk to wax lyrical- you'll wait for ever.
Worst thing that can happen is if no one shows, you get a nice sail in a swimming pool!