1670s, "one who or that which cruises," agent noun from cruise (v.), or, probably, borrowed from similar words in continental languages (such as Dutch kruiser, French croiseur). In older use, a warship built to cruise and protect commerce of the state to which it belongs or chase hostile ships (but in 18c. often applied to privateers).
Like the frigate of olden days the cruiser relies primarily on her speed; and is employed to protect the trade-routes, to glean intelligence, and to act as the 'eyes of the fleet'. [Sir Geoffrey Callender, "Sea Passages," 1943]
Meaning "one who cruises for sex partners" is from 1903, in later use mostly of homosexuals; as a boxing weight class, from 1920; meaning "police patrol car" is 1929, American English.