You might try Filters>Render>Clouds>plasma, play with the settings to get a fairly even texture and hit <OK>. Then use Colors>Colorize to give it an even blue coloring.

Then you just use the Dodge tool to lighten things around your coastlines. You can also use a larger brush to Burn some of the deeper areas.

It's just a thought although I'm more concerned with your inability to run Guassian blur. That sounds like you are on the wrong layer or something when you tried to fire it up. Unless you are seeing the bar begin and just hang because your machine can't handle it?

Anyway, hope it helps.