Dear Mitesh,
I’m not sure I really understand your questions.
Regarding MSL2SWL, normally this parameter is used to model water-depth variations as a result of tides or storm surges. Are you asking how to estimate these for a given site?
Setting initial conditions in FAST has been discussed many times on this forum e.g. see: Initial conditions for tower and rotor - #2 by Jason.Jonkman.
Best regards,