That's not the wording. I think you're getting gas confused with petrol.
This is the wording from MGN280 regarding hydrocarbon detectors for petrol engined boats:
7.3.2 In locations where an accumulation of hydrocarbon vapours is likely to occur, a suitable
hydrocarbon gas detector should be fitted under or adjacent to the tank (located in a safe
place). The detector components, and any other electrical equipment residing in the vapour
area should not be capable of causing ignition.
It's not at the surveyors discretion. If you have a compartment where fuel vapour could acumulate then you have to have to have a HC Detector.
If you've only got portable tanks on deck then there's no compartment where fuel vapour could acumulate so no requirement for a detector.