Grounding inside a plastic enclosure
If the screw is exposed out of the plastic box, it is an accessible conductor. If it is screwed with the Mean well AC-DC converter, the easy way I think is to make sure the converter meet safety standard. For example to check whether it has EN61204 safety standard certificate. If so then you are sure the converter grounding is okay.  Next is you have to make sure the screw contact well with the grounding. However, if you can loose the screw without opening the plastis box, you cannot just ground the screw. I would keep the screw inside the plastic box and do not let any metal exposed out of the box.

However, to make the box safe, grounding or not is not the only factor to consider. As you said you are using a plastic box, if there is not accessible conductor (aka exposed metel), you do not need grounding, just like the hair dryer in bathroom. Another important safety factor is the creepage and clearance between you electronics and the accessible surface (the plastic box surface). It depends on the working voltage level, rated impulse voltage level etc. For example, with reference to EN60335, the clearance for a common 230V appliance is 3mm. However as your product will be exposed in a salty environment, I will use the next higher level which is 6mm. The thickness of the plastic better be more than 1.2mm. And even better is you have double insulation, that means you have two plastic box with total clearance of 6mm.