As Quillan said
Did you recieve any damage and then repaired it?
There is a 'bug' where if you recieve damage to the batterys and later they appear to be repaired to 100% yet will only take on some percentage of full charge. Lets say 90%.
This causes your battery charging system to run continusually wreaking havic with your 'milage'. To stop this you have to manually select 'normal propulsion' after every time you dive. You can check the battery charge state by hovering the cursor over the 'gauge'.
This is the most likely scenario as fuel leaks are not modeled in the game.
Also when going submerged for whatever reason cut your speed to ahead slow. That way you can run for several hours without totally draining the batterys and therefore taking less time to re-charge.
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