Although the Nikonos is humidity and dust and water tight, it's not humidity and dust and water proof.. The difference may be subtle, but it's crucial to understand. It's the outside that's built to withstand the abuse, not the inside.
Especially a camera that's taken underwater will experience big temperature drops that may lead to condensation. So even if it's not leaking at all, moisture may form on the inside, and given that the camera is water tight (so there's no ventilation at all) may stay in there for long times if the camera is kept closed after use. If there's a small leak (or you haven't greased the seals) and water seeps in, so much the worse, especially because surface water contains spores, algae, and other muck.
Now, anything that's kept in a humid environment for a prolonged time will see development of fungus (when above water) or algae (when under water). Therefore, you need to clean and dry the camera after use, and allow it to completely dry and ventilate on the inside..
So, long story short, storing a Nikonos in a dry cabinet is a good plan, and keeping it in a bucket of water isn't..