To work offshore, on the rigs as a crane operator you would ideally need your stage 3 (sparrows or OPITO). The stage 3 allows you to perform lifts between the boat to the platform and on deck, a stage 2 is deck lifts only and the stage 1 is just an introduction to crane operations and nothing more.
Any crane operators course you do on the beach (CITB), and qualification you acheive is only recognised on the beach. They are not transferable to the offshore industry, you would have to attend the offshore equivalent ( sparrows or OPITO) courses.
Any experience you have operating cranes on the beach would come in though, the physics are the same.
If you wanted to go down this route (offshore) and gained your stage 2, you would find it quite difficult to find a position as a crane operator on a rig.
They mainly require the stage 3 operators, which in turn is pretty much unobtainable as an individual. You need to be assessed offshore (sponsored by your company) carrying out boat lifts ect. Once you have you stage 3 though you are able to take the assessment's ( as an individual) to maintain your qualification.
In my experience it is normally riggers that get to move into crane's, mechanic's too.
However, you are able to work on boats ( such as DSV's) with a stage 2. Normal day rate for a 12 hour shift is £350. This can vary though.
Also for NEBOSH General Certificate try these
SHEilds eLearning
Maybe try distance learning, if you put the legwork in there is no reason you won't acheive the desired results.
Regards
Skyhook