Just to give some answer...
Could flexing of the enclosure ever occur? Either in assembly (tightening fasteners), or in the field? Then you want to use a sealant that stays flexible.
Automotive silicone is what I think of. Engines are an extreme environment. There is vibrating, high temp, low temp, high pressure, solvents, etc.
For underwater exposure, use the type made for marine equipment.
If it will be subject to high pressure (or vacuum) then consider plumbing sealants. Screw threads will need a sealant such as thread-lock compound or teflon tape.
If it will be subject to high temperatures then use the type made to go around engine parts (or on the exhaust system).
Don't know about cold temperature limits.
Consider whether the enclosure must be opened in the field. Will a technician be able to remove old sealant, clean all surfaces, and apply new sealant? Consider making a gasket of suitable material that will be easy to remove and re-install.