Do fish depend on plants for survival?
Yes, photosynthesis provides nearly all of the ocean's biomass and energy. The best is to study oceanic food chains and webs.

It looks like there aren't enough plants in the ocean because they don't form huge reserves of foliage and branches... 10% of land plant mass dies and is replaced every year, and 100% of oceanic plants die and are replaced every week:
&quot;Biologists set out to estimate the total biomass of phytoplankton and calculated that less than one billion tonnes of the single-celled microorganisms were alive in the ocean at any one time. There were 45 billion tonnes of new phytoplankton each year, 45 times more than their own mass at any given time. The phytoplankton would therefore have had to reproduce themselves entirely, on average, 45 times a year, or roughly once a week. In contrast, the world's land plants have a total biomass of 500 billion tonnes, much of it wood. The same calculations showed that the world's land plants reproduce themselves entirely once every ten years.
https://www.nature.com/articles/483S17a
Every step of the marine energy pyramid has about 10% efficiency, so the shrimps and zooplankton take about 10% of the phytoplankton energy, then small fish 1%, and big fish 0.1% of the plant energy.
For every 500,000 units of radiant energy that hit the ocean surface, 2% is converted to phytoplankton at the base of the trophic pyramid, and the 500,000 units are necessary for every 10 units that a tuna consumes. https://oceantracks.org/sites/oceansofdata.org/files/energytransfer.png
https://en.wikipedia.org/wiki/Marine_food_web
In biology, we typically study coccolithophores and phytoplankton to learn the marine food web.
Areal map of sea chlorophyll in Mg/m2.