rip-rap n.

"rough water, ridge-like water," 1775, perhaps a special use of rip (v.); also compare rip-rap. Originally of seas; application to rivers is from 1828.

Hence rip tide (by 1862), which seems at first to have been applied to strong tidal flows, as in the Pacific Northwest. An 1896 letter from Alaska, published in a California newspaper, describes a rip tide as  "a rapid tide against a strong wind, producing choppy seas and an undercurrent, which renders a small boat unmanageable." By 1907, with the rise in popularity of ocean bathing, it came to be applied to dangerous intermittent strong currents flowing straight out from shore and can drag even strong swimmers to death by drowning. For this, precisians prefer the more accurate rip current, introduced for the purpose in 1936, but rip tide remains the popular term.