All exposures were with the same contrast filter. I use an old D2V with condensor head so the filters go in the condensor "drawer". I've always been paranoid about changing filters for fear of moving the head/inducing vibrations etc... But thanks for the tip, I may have to get over my fear and try your suggestion next time round.
I had a couple questions regarding the burning sequence you've described. Are you using the same contrast filter as the main exposure for the burns? It looks like the foreground water was burned in using a lower contrast filter to bring down the highlights without pushing the tones in water down too far. Retaining detail in the background, which works here without competing with bright rock textures, can be achieved this way too.