[Iron Budokan]

That's true. Even a sunny day affects the salinity of the ocean, driving off evaporation and increasing the salt content, thereby affecting the specific gravity of the sub itself. In Das Boot (the novel, not the film) Buchheim tells how every day they tested the density of the ocean water because it had such a big factor on trim weight.

It's much more complex than just flooding trim tanks, that's for sure. Much more complex than I originally imagined....a LOT of chemistry and physics involved. Pretty cool.