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> that green light doesn't penetrate water as well as other wavelengths

As a diver that sounds weird. Red is the color that disappears quickly with depth. Blue is preserved the most.

Additionally, land based plants have had some serious time to change color if more energy was beneficial, which clearly must be the case?

> there was no advantage

Well there is one disadvantage even: overheating.

This doesn't explain why terrestrial plants didn't evolve some way to utilize e.g. UV radiation. It should be possible in theory, since we know of radiotrophic fungi: https://en.wikipedia.org/wiki/Radiotrophic_fungus

The UV frequency range has much less energy in the solar spectrum than visible light, see here: https://sunwindsolar.com/solar-radiation-spectrum/

Ironic if those are the result of mutation caused by the UV radiation. If UV causes cancer in humans it's probably not good for fungi DNA.

I think some indoor gardens do add low UV but I don't know exactly

John Lydon did some research with UVB radiation and cannabis that you might find interesting. This was done back in the 80s.