Strapless, bandeau-style swimwear is by far the best style for tanning at sea. I can sit in the sun or snorkle all day and forget about nasty tan lines. I particularly love my new double-face bikini, it's navy one side and emerald green the other side, you can buy them in many beach towns in Italy but haven't found many elsewhere. I also love the two below. It doesn't come more nautical than underwater for todays pic (courtesy of my sister's new camera).
Zimmerman Corset bikini £165 and Lisa Marie Fernandes £300 both at netaporter.com