This is the second idea in the works at the moment that plays with the expansion and contraction through passive means.
Using the Hoberman Sphere, this viewing platform expands and shrinks with the rise and fall of the tides. An air tight appendage is extended into the water and set at a given boyancy that places it at a set depth controls the size of the sphere.
The appendage works on the same principle as a submarine through ballast tanks and can likewise be found in several creatures in nature such as the Portugese Man O War.
More exploration into this is being made...