Hi Marc
I'm not sure what you mean by "floods the whole scene", my guess is that you have an ocean that goes on to the horizon, is that a red sea in your picture?
I had Bryce packaged with my new PC, I loaded it up and had a play, not an expert or anything.
Your land pieces are scalable objects, and your camera is movable, so if you took one of the islands, stretched it to be a lot (even 50x) wider, possibly pull and push it in the other axes too if you want, and place it a lot further back from camera, it will give you a large landmass, and your water plane can become a lake or several.
There are some example scenes in the install directory under Scene Files>HDRI Samples and I found it helpfull to load up these scenes and then play about with them.
I made a Hoverbout in Lightwave:

and then played with one of the sample scenes adding water and deleting bits to make this:

I didn't show it to anyone because the Dalek is Rob's and the scene came with Bryce. It was just some experimenting but it highlights what I meant about putting land between the water and the horizon.
My 2 gripes with what I had done were that Metallic and luminous surfaces from LW did not come out in Bryce (I can't find a pic of a more recent LW render but the Hoverbout had a glowing underside and was shinier but my 3D machine is in the bedroom and it is late here). It might be something I did when importing, but I couldn't find any relevant options.
The second was that smoothing is applied by object, not by surface, so you cannot tell from the distance, but those Daleks have rounded skirts, again may just be me that's not very good.
By the way, the Hoverbout design is based on their first ever appearance in Century 21, the Dalek colour scheme is from a TV Comic story.
Anyway, hope that something in there helped.
Night from me,
Dave.