Floats of Algae, a house-made conjecture
One consideration is the energetic costs to the organism. It's going to be energetically more favorable for an organism to be in osmotic balanced (isotonic) with its environment than to constantly work to maintain lower internal osmolarity through active transport of water or solutes. Second thing to consider is cell size and density differentials. Air at sealevel is several hundred times less dense than water, so even tiny gas bubbles would create significant buoyancy for a cell. On the other hand, pure water is only about 2.5% less dense than sea water, which means a much (much) larger volume would be needed to generate equivalent buoyancy.

All that said, there seems to be some plausibility to your conjecture, as there are some large marine phytoplankton that use active water transport to maintain vacuoules at lower density than that of seawater (Source). 

This paper discusses some other competing mechanisms as well, but as the author of the article states: 


  "... most of the cases of positive buoyancy in phytoplankton not
  generated by gas vesicles concern diatoms..."


That's not an incontrovertable refutation of your conjecture, but it seems to me that gas vesicles are a much more efficient means of generating buoyancy, and would probably be energetically favored over other biological means of generating buoyancy.