reef (n.1) Look up reef at Dictionary.com
"ridge underwater," 1580s, probably via Dutch riffe, from Old Norse rif "ridge," literally "rib" (see rib).
reef (n.2) Look up reef at Dictionary.com
"horizontal section of sail," late 14c., from Old Norse rif "reef of a sail," probably a transferred use of rif "ridge, rib" (see rib). Cf. German reff, Swedish ref, Norwegian riv, Danish reb, all from the Old Norse word. Reefer as a nickname for "midshipman" (1818) is source of the meaning "coat of a nautical cut" (1878).