Slitherine

How do we know when a sub needs to surface to recharge its batteries? I don't see a number like planes showing how many turns they can stay in the air.

And, how long does it need to remain surfaced for a full charge?

Or, is this just flavor text?

I also haven't noticed any drop in efficiency so far when submarines keep submerged a dozen or two of turns in a row. Just keep snorkling around!

Hm were does it say this? Subs used to have a mechanic similar to aircraft fuel but we removed that limitation months ago.

tooltip_switch_submerge_disabled_2 = A submarine cannot submerge until its batteries are recharged.
tooltip_switch_surface = Surface submarine to recharge batteries.

Yep. There.

I guess the tooltip text was just forgotten and never updated. Poor neglected tooltip text.

Naval efficiency is a little bit strange, it "resets" it's self each turn (as I understand it) so subs keep plodding along taking 1 point off the odd ship every so often until a destroyer ends it's turn on top of it (so getting to dump around 4 points of woe on it ... who needs sonar )
A submariners lot is not a happy one .. at the moment.