The online surfski community recently had a discussion on this subject after a long swim by a paddler whose white boat was not seen for several hours. The conclusion was that bright pink was the most visible color on the open sea, followed by yellow. This follows my subjective experience. White and black boats require careful observation to spot. Subtle colors may make beautiful boats, but can blend into the background of chop. For the easiest spotting, you want a color that pops the boat from its surroundings. I thought my bright yellow ski was rather garish when I bought it used last year, but I've grown to love it and value its visibility.

Also, remember that the hull my be the most exposed part of the boat in an emergency and you should consider its visibility as well as just the deck.