non-motile phototrophism[ like angiosperms]
i think its required by non motile forms of phototrophs more…
i think those floating weeds don’t need it… might need also because in a crowded pond, may have some kind of it…but in sea it might be there to make it grow horizontally…but ithink less chances as those weeds with horzontal natural growth pattern will not face any competition… those land plants may be the orogins… underwater plants that are stuck to the bottom have no definite tendacy , they just let their heads flow!

motile phototrophism [ of bacteria and all]-
must be evolved in muddy forest ponds where light patterns keep on changing…due to changing tree-shadows…same possibility underwater…
under the weeds or any other motile-non-motile thing which has a changing shadow pattern…
for if it evolved under water or not- check if it is triggered by a specific wavelength… if its lower one like blue or so… then under water where blue can’t reach…such a mechanism can’t evolve…if red is a trigger then
we need more tests…

Good luck! communicate ur findings…curious!

actually from what i know the movement of bacteria, protists, chloroplasts within a plant cell influenced by light is not called phototropism, it is called a phototactism

So, we need to consider only non-motile part…thanks.

Most plants exhibit phototropism. But in biology, there are exceptions to everything..