It would be sheltered from gamma radiation. Even a few tens of centimeters of water is enough. (Phytoplankton is dispersed throughout the upper 50 meters or so,

It would be sheltered from gamma radiation. Even a few tens of centimeters of water is enough. (Phytoplankton is dispersed throughout the upper 50 meters or so, though it's concentrated closer to the surface.) UVB from sunlight after the gamma ray burst strips away the issuing layer is what people are referring to. Because it's longer wavelength, it could still have an effect.