Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1200, creulen, from a Scandinavian source, perhaps Old Norse krafla "to claw (one's way)," from the same root as crab (n.1). If there was an Old English *craflian, it has not been recorded. Related: Crawled; crawling.
1818, from crawl (v.); in the swimming sense from 1903, the stroke developed by Frederick Cavill, well-known English swimmer who emigrated to Australia and modified the standard stroke of the day after observing South Seas islanders. So called because the swimmer's motion in the water resembles crawling.