Ah ok the reef, now that I know where we are I can help a bit to point ya into another method of experimentation. Render up some green to blue clouds then cut em down to keep em close to the land. The layer's mode was set to normal but an additional layer on top of the reef was set to color dodge, thus lightening and brightening things. If GIMP doesn't have color dodge then just render up something lighter or use some black n white clouds as some other type of adjust, like screen or lighten or overlay or soft light. The bottom line is all about experimenting to see what you like best. I myself just went back the other night and made a whole bunch of tweaks to the tutorial based on nothing more than changing the way I did things to see what would happen and if I liked it then I added it in (I spent about 3 hours trying to create some of those cool white waves around the coast but didn't like anything).