I just found another one, to the South West, except its not strictly a seaplane base, despite its runway being clearly over water.
Lake Cushman Dock (LCMD)
It’s in italics, so is considered an “addon” airport in LNM, and has a single water start.
Change the view to only show Seaplane Bases:
The ones with yellow circles then stand out.
Even stranger, when you select LCMD from within the sim you can’t actually start there. The Fly button is greyed out, no matter which float or non-float plane I try.
But if I spawn nearby then teleport to LCMD:
Must be broken, possibly even the wrong designation, and it should be a seabase and not a soft surface strip as LNM classifies it, which might explain why I can’t spawn there.
And surprise, surprise its from the same bushtrip I listed above:
D:\Flight Simulator\Official\OneStore\microsoft-bushtrip-sanjuan\Scenery\bushtrip-sanjuan\Scenery\San Juan.bgl
I know you get some red text warning you if you try to spawn at a seabase in a non-float plane. I’m guessing that it won’t let you spawn at LCMD in a pure float plane because its a seabase, but thinks its soft surface, and the internal logic doesn’t allow for generating an error on screen for that configuration?
In any case, C&D water starts are definitely a thing, though you may need to go out of your way to find them. 