[Bronzewing]

I agree with most of the points made already. so I won't repeat them. I have some more ideas though.
1) Tides. I think it'd be nice for work in shore if you had to consider the tide state before trying to cross shallow water.
2) A round world. First time I tried crossing the Atlantic in a type IX I noticed that SH3 world appears to be flat. I plotted a grand circle course and it was longer than a straight line. :P
3) More serious battery damage effects. In SH3 battery damage is an inconveniance. IRL it was potentially deadly as sea water in the batteries made them emit chlorine gas into the boat! Having to deal with gas leaks could make things quite a bit more interesting.
4) Fires actually damage ships/aircraft. I've had burning aircraft circling my boat for up to 30 minutes or more, is this realistic? Not very. Also if you started a big enough fire on a ship like a tanker, even if it didn't sink the fire would destroy it. One time in bad weather an Elco MTB found my boat and forced me to dive. The Elco had a HUGE fire burning aft the whole time but the AI completely ignored it!. It finally left, doing 20 knots, STILL burning, over an hour later.