SWL - Still water level
Still water level is the elevation (level) of the sea including Tidal level and MSL, i.e. the level of the sea with the motions such as wind waves averaged out..
Read about SWL in the Wikipedia article about Sea level or in Coastal Wiki.
Both Tidal level and MSL parameters are by default zero. So, the SWL is also by default zero.
Note the difference between the terms SWL and MSL: in the former water is used in the latter sea is used.