I'd heard that Crabs were so called because when they were given there uniforms, (once destined for pre-revolutionary Russia - I believe), they were the same colour as a lotion used to cure pubic lice!
Not sure if I'm falling for a Waaaahhh here, but here goes:
Crabs short for crab fat, because all those shot down pilots in WW2 made the crabs in the English Channel fat
Matelow is from a French word meaning 'bed partner', as there were only 1/2 or 1/3 of the number of beds on board ship to the number of crewmen. You slept in shifts and one bed was allocated to several crewmen.
The above info is my belief - I stand by to be told otherwise.
[French, from matelot, sailor, from Old French matenot, sailor, bunkmate, possibly from Middle Dutch mattenoot (perhaps from matte, bed from Late Latin matta; see mat1 + noot, fellow) or from Old Norse mÃ¶tunautr, messmate (mata, food, mess + nautr, companion).]