You can use a light blue buttercream with white RI "foam" on top or you can tint piping gel blue to get the effect of water.
I use blue BC, then add a little darker blue gel over that. You can also add a little white BC here and there for foam.
Karen
If covering the cake with fondant you can tint fondant a couple shades of blue, mix the two shades together a little - so the colour is still a bit swirly/patchy...cover the water area with that, then paint over it with a thin layer of piping gel to make it look wet. Add a bit of white royal icing for 'foam' in places if needed.
I used piping gel over fondant on this cake: http://cakecentral.com/gallery/2020202/2020209
Oh, that is the coolest cake, Coral3! I LOVE the way you did the water in it. I love the way you did a lot of other things, too, but since this thread is about the water, I'll stick to that. Very original - I've never seen a surf quite like it before. Such a good idea!
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