I use Semco with fairly good results in brackish water. It does gets easily blotched when saltwater is over the bow but still keeps the decks from turning grey even with a bit of 'blotching'. Usually lasts 2-3 months but has to be double coated.
My current 'mix' is 1/3 Semco Natural, 1/3 Teak Wonder, 1/3 Olympic (carmel) deck sealer ... 3 coats will get me through 6 months and green water doesnt blotch it as before. Best of all these are 'water wettable' and you have little loss with deck shoe to deck adhesion when the deck is wet. If needing to 'repair' any salt water blotching, just brush on some oxalic acid which will soak through the coating to the wood ... and without disturbing, etc. the coating.