Pretty easy if you look at nautical lingo: captain is taken from kapitein, also anker/anchor and lijn/line, etc. My favorite etymological explanation:
Back in the day the rudder had not yet been invented, so they used a slightly oversized oar to steer (dutch: stuur) the ship. It was not practical to fasten it exactly in the middle, so it would have to be either on the right or the left side of the ship. Because most helmsmen were right-handed, the oar usually got fastened to the right side, which was henceforth known as the stuur-boord, literally the board of the ship that the steering oar was fastened to. Because the uu-vowel doesn't do too well in English, it got turned into starboard, which is still used today. Coincidentally, because the steering oar was in the way when mooring the ship it became customary to moor the ship on the 'left' side. This then became known as the 'port' side because the port was always on that side.