What makes a ship float is the fact that it weighs less than the water it displaces. This is because it is hollow and filled with air. Replace the air with water and, well, if you ever get the chance drop a steel bar into the deep end of a pool sometime; see how fast it sinks to the bottom. Even wooden ships sink if you make holes in them, and they're made out of a substance that floats on its own.
As for electronic digital calculators, the first one wasn't made until the mid 1960s. The first pocket calculator wasn't made until about 1970, and cost around $400.
What the submarine captain in WW2 had at his disposal was the whiz-wheel, or circular sliderule. Hitman has made one you can download, assemble and use by hand, or joegrundman put one in his U-Jagd Tools mod, and OLC included it in his GUI mod. Both of those are for SH3, but Hitman's can be used with any game, because it's hand-held.
A digital calculator might be handy, but it's not something they had at their disposal back then.
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