I was very heavely into marine aquariums for many years, I did a talk at Chester Zoo regarding this subject and have had a paper published on propogation of soft corals.
Water and the risk of shock should always be limited, some are of the opinion that a RCD could trip without the owner knowing and the loss of thousands of pounds of stock frightens them more than the possibility of a shock or even death.
I have put my hands into tanks and felt a belt similar to those you get with static, and yet the fish and corals are or appear to be ok. Heating filtration and anything else can actually be done outside the tank, which is the preferred method for most marine set ups.
On a personal note I would RCD.