Depends. Where are you operating, geographically?
Some parts of the world use QFE in the airport environment, QNH when clear of controlled airspace and standard passing through the transition level (quite low some places). And that's just the baralt.
Others use QNH throughout and one may never go through the transition level and an offshore platform may or may not offer a particularly accurate QNH in some places.
Some bits of offshore water in this world aren't at sea level or even particularly close to it, in which case offshore gezza's method can get one in a world of trouble. In those cases, it's advisable and usually procedural to use the radalt while leaving the baralt at local QNH, at least once one commences descent. Most offshore IFR approaches are predicated on the radalt. This will all be spelled out in one's SOPs.
In transitioning from one platform to another in the same body of water, one usually uses the same altimeter one was using for the first one for the second one.
So, it depends.