[adrianratnapala]

I thought even today phytoplankton produced more oxygen than all the multicellular plants put together.

I might just be imagining that factoid though.

[HCIdivision17]

That was my understanding as well. The idea being that there is a lot of volume in the first few meters of ocean surface area churn. It's not hard to imagine that does better in surface area than (even a lot) of plants.

[ethbro]

Not forgetting the fact that it seems like it would be substantially easier for a faintly mobile fluid filled bag(s) of DNA to survive and reproduce in an aqueous environment!