Depths have been an integral part of coastal navigation for centuries. Sailors used a "lead line" as far back as the ancient Greeks and Romans. In addition to depths, the type of sediment on the sea floor was also catalogued. There was a cavity in the bottom of the weight, usually filled with tallow (rendered animal fat), which would pick up sediment upon hitting the bottom.
When in well-traveled coastal waters a sailor's lead line showed (for example) ten fathoms and sand, he would have a fairly good idea of where he was in relation to the coast. Admittedly, knowing the type of sediment below is not very useful in a submarine ... but moderately accurate depths would undoubtedly have been available for the entire period of their use.
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