Excellent! I have been on the Royal Soveriegn many times, we used to change the crews every 2 weeks using a Bolkow 105D out of Shoreham. Brings back fond memories, indeed, I look back on the flying I did for Trinity House as some of the best flying of my career, it was a privilege to have done it.
As for the RS, the deck is a decent size so you should have no problems. There seems to be a big myth about landing on offshore platforms, if you can execute a decent approach and landing to a helipad onshore then doing the same offshore should present no problems in daylight and good weather.
You will of course be single engined over the sea and will need to carry the appropiate survival equipment. It would be a good idea to have it clear in your own mind what actions to take should you be unfortunate enough to have to carry out an autorotation to the water.
Sorry if that last passage sounds a bit gloomy but I do hope you enjoy.