Unlikely that there is much truth in that statement about coolant and lubricant mixing.
More likely they did not need antifreeze in the engines because the sea water rarely gets below 0'C and in those cases the engine would most likely be running.
Once a submarine dived, the water temperature would be above 0'C because water from 4'C to 0'C gets lighter as it cools (that is why icebergs float)

__________________
Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.