Hi PacChill
The 5 lakes around fuji are know as the Fuj Go Ko (the fuji 5 lakes) and there is diving in one that I know of. I have dove once in Motosuko. That lake has one of the nicest views of Mt. Fuji. It used to be featured on the back of the JPY 5,000 bill, fuji reflecting on the lake perfectly. But it was replaced with the head of someone I am unfamiliar with.
Anyway, The lake is deep in some places and at altitude, so modified dive table is in effect. The bottom is silty so good fin kicks are a must. When I dove there, all I saw was a lot of quite large bass, somewhat interesting, and tons of snagged bass lures on the bottom. I suppose I could have made a nice tidy sum if I had collected and resold some of them. The lures sell new for 20 to 30 dollars each.
I can help you out withe specific details of diving there if you are interested (PM).
Jim