lamont:
The PBM Mariner float plane is at 70.
It is, but IIRC, the Cedar River dumps in right there, and it's buried in silt. Isn't that the one that the navy tries to dig it out every so often and then gives up on when they manage to hurt themselves in the salvage process?
Most of the people who know about stuff in the Lake protect the GPS coordinates for the sites fairly well, to discourge people from damaging the artifacts, and to discourage those not qualified for the depths from going beyond their limits. If you know the coordinates (or have friends that do), they're interesting places to dive once or twice, but that's about it.
Last time I did the PB4Y off of Sand Point (significantly below recreational depths) I had about two - three feet of viz, and the 21W HIDs weren't doing a whole heck of a lot. Still, it was one of the coolest dives I've done...
I had a splash in the spring in Kirkland (there was an old shipyard years ago around Carillon Point, just north of the marina, and there are a lot of stumps there from turn of the century logging.) I was just checking out some borrowed gear in a very shallow 30' area and I had about 10 feet of viz, making it a "good day." We chose that site mostly because my dive buddy was in fresh water that day, teaching a trimix class, and it was easy to meet up there.
There's not a whole lot of stuff down there in Lake Washington worth seeing outside of the wrecks of the planes/barges/coal cars/minesweeper/etc, especially considering that Alki/Edmonds/Three Tree are so close and convenient to even the Eastside. Compared to the salt water, the lake's fauna is positively barren. Do the lake once or twice just to say you've done it, and go back to enjoying the diversity of critters down in the salt water
There are a few other "recreational" sites you can do fairly easily in the lake without having to have a boat, know where the coordinates are, etc - the I-90 bridge has sections sunk in fairly shallow water (although the cables and bridge above and boat traffic might make some people consider it a bit "advanced.") There's a clay wall off of Leschi I've heard is interesting, but I haven't had a chance (or the desire) to dive it myself.