Only just realized this when watching a diving video.

But why isn't the ocean full of massive plants? The surface of the earth is covered is giant trees working to fight against gravity, wind, and fire.

But in the ocean you have little things like coral beds and sponges and maybe a kelp forest.

Seeing as weight is massively reduced, and the closer you get to the surface means more light, and water and nutrients are probably plentiful... Why isn't the plant life more massive? What is the limiting factor to things evolving to be bigger?

Only thing I can think of is CO2 perhaps?