I am trying to create the appearance of water atop a sponge cake. My first thought is to use swirled blue colored buttercream, but that is hardly novel or original. Could I achieve a marbled appearance somehow to simulate whitecaps on waves? Some other technique to simulate water motion? Fondant? Any suggestions appreciated.

if you are feeling very adventurous, you can do stuff with melted sugar....
a lot of people like using blue piping gel. blue royal or bc works too, you can just rough up the surface for waves. (marbelling is a good idea, but the problem would be white in the right places)
fondant could be done, you could paint on the white, especially if you make the fondant lumpy (like balls/ ropes of fondant underneath it)
xx

i usually use a stiff royal icing for water, in a variety of colours, dark blue light and white, start with the dark and mix the colours as i go, sort of swirly movments with my spatula so they become marbeled but not to mixed up, then i added a few blobs of white to make peaks and crashes of waves

I've made sugar look like water. I just followed the recipe for rock candy. I poured it on top of wrinkled aluminum foil to give it texture. You could flip it right side up or wrong side down. You could just make one and when it starts to cool down you could use a knife and try and pull it up to look like waves. On top of that you could use icing to make white caps.
If you use piping gel you could put some tonic water in it. If they turn the lights off it will glow.

Thank you all for your replies. You've given me many great ideas!

i found gel icing to work really really well! it gives it a really life like apperance because i is still somewhat clear and has a "wet" apperence. its very easy to make yourself as well. =)

I read somewhere to make sure underneath blue piping gel is white otherwise the other colour shows through and interferes with the blue.

That tonic water idea rocks!!

I've coloured grapefruit jam (it was the lightest colour I could find) with blue food colouring before. It looked great and thought it tasted much nicer than a piping gel.

Lyndsey xx