My Merriam-Webster gives the etymology thusly:
Afrikaans, from Dutch, a kind of monkey, from Middle Dutch meercatte monkey, from meer sea + catte cat
Date:1801

Which suggests that the phrase "sea cat" meant "monkey" in Middle Dutch, so the little critters now called meercats were so named not because they resembled the sea, or cats, but because they reminded someone, presumably someone who spoke Middle Dutch, of monkeys. That's a resemblance I can get, but I don't get why "seacat" should mean "monkey"