Source, The IPW Museum book of The War at Sea,, British submarine operations and the conditions in the tropics. Story told by Petty officer Ian Nethercott. A young South African Sub Lieutenant went to the Coxswain, and said he`d got a dose of the crabs . The Coxswain said "just sit over a bucket of salt water" "What does that do ?" "Well" replied the Coxswain " they keep looking down at the water and as its moving about with the motion of the boat, they get dizzy and fall off" So he`s sitting on this bucket, with his trousers down, when the Captain comes in, and says "What are you doing subby ?" He told him "Yes" says the Captain "But that takes a long time. The best thing is to go and get a drop of rum from the Coxswain, mix it with sand and rub it in" "What will that do ?" "They get drunk on the rum, throw rocks at each other, and brain each other"
Not sure what "crabs" are but my guess is that they are the equivalent to "Sackratten" which also make an appearance in "Das Boot".
Crabs are pubic lice, a sexually transmitted paracite, once met someone who had them in her eyebrows. Nice joke, nice find.