Check in the tutorials forum, there's got to be something in there on doing oceans/seas.

For me, I use several layers starting with a very dark blue and moving to a nearly white for along the coastlines. the bottom most (darkest layer) usually covers the entire map area. The folowing layers each gradually shrinking around the various land masses etc. With guassian blurring to help blend the colors together to be more natural. I often (but not always) use some kind of pattern/texture on the layers.

Not the greatest explanation I know...but it should at least get you started.