Hello Buki,
There is an “Attenuation” parameter in the FSO component that you could use for attenuation underwater. Also, there is another parameter “Additional losses” that can be used for extra losses that could be encountered. You may use the “Intensity scintillation” to count for environmental conditions encountered underwater.
I recommend you to check an experimental paper and tune the parameters to match the simulation with experimental (A synthesis exercise).
Regards,
Ahmad