I'm making a island of Jamaica cake this weekend. I want it surrounded by water and sand...What can I use for the water? I was tjhinking fondant on the board but I want it to have some texture. I saw some posts where isomalt was mentioned, are there any other things that I can do?

Piping gel tinted with blue and a hint of green--either spread it on with a spatula and shape into waves, or put down some buttercream waves first and paint with the coloured gel. 

If I pipe buttercream down first should it be white or blue?

I would use a spatula or palette knife or even a table knife to lay down the buttercream in waves with peaks. If the buttercream is the right (slightly soft)  consistency, the tips of your peaks will flop over and really look like waves of water.

I would use white, because then the gel over it shades to the right watery look without any effort at all. Thicker gel at the base, thinner near the peak.

If you take a little time to practise this on a plate, you will see that you can do a row of waves in white, add the gel, do another row of waves, add more gel...very realistic effect. To correct, just scrape off.

Add the sand after you have all the waves down on the board.

Great, thanks for explaining