Speaking as an oil painter who's spent some time observing such things the color of the sea depends a great deal on the color of the sky, so while you'd probably have some explaining to do if you painted your sea bright orange it wouldn't necessarily be inaccurate.
However you're probably better off taking the advice above and going with blues, greens, and grays... which specific ones isn't terribly important..  The key, I think, is to vary it a bit.  Paint a board one shade of blue and it's going to look like a blue shingle, but toss in a slightly different shade each time you empty your brush and you'll have something more convincing.