For this type of application, I'd suggest using a 'switch-mode' controller circuit. This type is highly efficient - very little waste heat is generated. There are has some different 'varieties' depending on what type of switching system is used - they have some subtle pros and cons. A basic Pulse-Width Modulation (PWM) switching unit will probably suit your application.
About the only drawback of this type of unit is the generation of high frequency 'noise' / emissions - most of this will probably not cause you any concern and can be shielded or filtered out if they do cause concern. As you want to use it in a harsh environment, sealing the unit in a small, fully enclosed metal box will prevent most 'interference' from occurring and give you a good degree of water/salt ingress proofing as well.
You need not specifically look for a light dimmer to suit your application, a DC motor controller will do the same job quite satisfactorily. There are many self-build and pre-built kits available for this type of application; Google should be able to point you in the right direction.
Hope this lot helps you somewhat - please let us know how you go.
CJ