LB, in case this hasn't been brought up already regarding Mercer Island and Bill Gates-

Ummm….don't think so. Bill Gates has home in Medina, on Lake Washington.

wikipedia.org/wiki/Medina,_Washington

Never heard of one on Mercer Island.

As to another list, Mercer Island would not have great access for subs as it's in a lake, with locks between it and Puget Sound. Lots of islands in the sound, but not Lake Washington.

Lake Washington and float planes, yes. Hydroplanes, yes. Canoes, yes.