In Florida saltwater, I would recommend a brighter color. I have a titanium blue and an orange kayak and can't tell any difference as far as being seen by fish. The main thing is being seen by power boaters. A kayak that blends in with the water is much more likely to get run over (believe me, power boaters are notoriously unobservant, particularly after a few beers).

If you have a rotomolded kayak, forget about painting the bottom. Paint will not stick in the long run and it will look like crap. You invariable scrape up the bottom on the beach, sand bars and oyster bars.