Will live power plug dropped into swimming pool kill the swimmer?
Water and power don't go together. Could it be lethal? The answer is yes.

A similar problem occurs at marinas and boat docks, I believe the last I read at least 5 people a year die each year in the US from stray currents causing involuntary muscle spasms leading to drowning.

Water breaks down the skin resistance and this makes us more susceptible to shock. Water is not a good conductor and distilled water is actually an insulator so while the electrons are trying to find their way back to the source, a body coming in contact with ground becomes a faster way back resulting in a severe shock and possibly electrocution. This problem in the stray currents is not usually line voltage, but a cord end in the water would be line voltage - 120v in the US, 240 on your side of the pond would be worse.

I believe your RCDs are set at 30ma with your feet on the ground this sounds safe to me, but in the water the effects can be worse because muscle spasms. On dry land the slight spasms many times remove you from the danger but in the water you can't get away and when you contact a grounded surface trying to get out the shock gets worse. In the US our GFCI devices trip at less than 6ma, all 120v outdoor outlets including ones around pools are required to be protected for this reason. Hope that helps.