  Aquatic photosynthesis plays a major role in carbon fixation and O$_2$
production on Earth. In this Letter, we analyze the prospects for oxygenic
photosynthesis in aquatic environments on modern Earth-analogs around F-, G-,
K- and M-type stars. Our analysis takes into account the spectral type of the
host star, attenuation of light by aquatic organisms, and rates of respiration
and photosynthesis. We study the compensation depth ($\mathcal{Z}_\mathrm{CO}$)
and the critical depth ($\mathcal{Z}_\mathrm{CR}$), defined respectively as the
locations where the net growth rates and vertically integrated net growth rates
of photoautotrophs become zero. Our analysis suggests that
$\mathcal{Z}_\mathrm{CO}$ declines by more than an order of magnitude as one
moves from the habitable zones around Sun-like stars to late-type M-dwarfs, but
$\mathcal{Z}_\mathrm{CR}$ decreases by only a modest amount ($\sim 40\%$). For
M-dwarf exoplanets, we propose that the photosynthetic red edge may constitute
a more robust biosignature of aquatic photosynthesis compared to atmospheric
O$_2$.
