Dolphin's sight only sense focus could be pretty strong. One sense we use all the time and take for granted is sight, often people hear and touch, smell and taste things without even realising it or eing aware of them; eyes however, we always know what we are looking at. I also guess that if focussing on only one sense as you fall asleep you are setting the importance of that sense up for your unconscious mind as it builds a dream for you, which in turn might work in the dream being first visual and then the other senses falling in around that. Sight being used more recognizably would then in turn be the thing that tells your mind 'hey something is happenning, check this out' and brings you to lucidity.

I am no expert but this would be the way I see it working on paper.