This site is filled to the gills with map samples and tutorials. I suggest you examine the various world maps in the Regional WIP maps threads for starters. A number of GIMP users make excellent oceans.

There's lots of ways to go, I don't know which style will appeal to you most - I like knowing where the shallows are, where continental shelf is located and where the abyssal plains are found using various hues of blue, but that's just me. Check out the site and find the style that works best for you...