"small open vessel (smaller than a ship) used to cross waters, propelled by oars, a sail, or (later) an engine," Middle English bot, from Old English bat, from Proto-Germanic *bait- (source also of Old Norse batr, Dutch boot, German Boot), which is possibly from PIE root *bheid- "to split" (Watkins), if the notion is of making a boat by hollowing out a tree trunk or from split planking. Or it may be an extension of the name for some part of a ship.

French bateau "boat" is from Old English or Norse. Spanish batel, Italian battello, Medieval Latin batellus likewise probably are from Germanic languages. Of serving vessels resembling a boat, by 1680s (ship for "serving vessel or utensil shaped like a ship" is attested by 1520s). The image of being in the same boat "subject to similar challenges and difficulties" is by 1580s; to rock the boat "disturb stability" is from 1914.

mid-12c., swein, "young man attendant upon a knight," from Old Norse sveinn "boy, servant, attendant," from Proto-Germanic *swainaz "attendant, servant," properly "one's own (man)," from PIE *swoi-no-, from root *s(w)e- "oneself, alone, apart" (see idiom).

It is cognate with Old English swan "shepherd, swineherd," Old Saxon swen, Old High German swein, but not directly related to swine. Also in Middle English "a common man." The meaning "country or farm laborer" is from 1570s; that of "lover, wooer" (in pastoral poetry) is from 1580s.

a mid-19c. respelling to reflect the modern pronunciation of boatswain.

Proto-Indo-European root meaning "to split," with derivatives in Germanic "referring to biting (hence also to eating and to hunting) and woodworking" [Watkins].

It might form all or part of: abet; bait (n.) "food used to attract prey;" bait (v.) "to torment, persecute;" bateau; beetle (n.1) "type of insect; bit (n.1) "small piece;" bite; bitter; bitter end; boat; boatswain; -fid; fissile; fission; fissure; giblets; pita; pizza; vent (n.).

It might also be the source of: Sanskrit bhinadmi "I cleave," Latin Latin findere "to split, cleave, separate, divide," Old High German bizzan "to bite," Old English bita "a piece bitten off, morsel," Old Norse beita "to hunt with dogs," beita "pasture, food."