What you said;
There will most likely be some ancient fish down there, and or ancient microscopic organisms that will help explain the orgins of life and what not.
But just saying seaweed or bacteria at best is a understatement.
Is highly incorrect.
It is not likely that there is a big ancient fish down there, or anything to help explain the origins of life.
Seaweed and bacteria is pretty realistic (although seaweed may be a bit wishful, considering there is no light down there, so seaweed would most likely not flourish).
There is an infinitesimally small chance that there is 'some ancient fish' or 'something to help explain the orgins of life'. Not 'likely'.