I'm not really sure how the great stuff will handle being submerged long term. If you seal it with silicone that can only help though. The real problem might be keeping it submerged, the stuff is going to want to float. However, if you add enough substrate/rock/driftwood atop it, you should be able to hold it down.
Are you planning on using great stuff for the background too? If so, I would recommend spreading a layer of silicone over it first. A rougher surface will help it stick.
And don't make the mistake I did... Leave the false bottom in when you create the background, or at least, be sure you will be able to fit it back into place after it is done. I didn't think of this and it was a PITA to torque the false in there after the great stuff cured. Luckily, diffuser is a little more flexible than one might think.
This is looking good though. I cant wait to see the finished product. Any thoughts on inhabitants?