HYCOM runs on a earth sized sphere, and 0m is the surface of that sphere.
By water level you may mean sea surface height, which is positive upward and again w.r.t. the sphere surface at 0 m SSH.
GOFS global mean SSH is constrained by climatological T&S, and this means that its long term global average is very close to 0.0. The observed SSH has a sea level rise of about 3 mm per year, and this is *not* in the GOFS SSH.