When a player choose the Captain role, then all players may participate into the sea battle, and the plundering afterwards?
Yes, all other players may participate in offered battle. If a player does not give any of his ships, he gets a black spot.
If multiple players choose Captain, then we repeat the sea battles, rolling of dice, and loading of goods, again? Clockwise?
Yes, after resolving the first battle, next player in clockwise, who also choose Captain, plays his card and new battle goes on, etc.