The information Rentsch is referencing is not current.
There are 3 "Depth States" for Subs.
Surface
Periscope
Maximum
There is no functional difference between 30m and 60m in terms of gameplay mechanics if both are within the same Depth State of "Maximum Depth".
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If you aren't interested in playing a Submarine on live server, please try one in the Submarine Operation on PTS. The mechanics are quick to pickup with a few plays.