In addition to the advice already given, it's essential that you take advice from both Blackpool ATC and Bond/Scotia Helicopters who operate the heli' rig flights in the Irish Sea.
Flying over the sea in the scenario which you're suggesting is very different from doing it over land if you're not used to it, especially at lower levels.
Quite apart from the rapid changes in the weather, the structure of the rigs themselves can often lead to unexpected turbulance in the slightest of winds. Also, be aware that birds can occasionally be a problem, and are more difficult to spot against the background.
Apart from that, you should get some sectacular shots of both the rigs, and if you're lucky, some of larger marine life which transits the area, enjoy.