A and C,
With all respect, I would feel uncomfortable transmitting a mayday at the last minute. For the reasons that you describe, it appears to be a good idea, but if you weren't heard for whatever reason by another flight crew, nobody would know the position of the aircraft.
Surely a better approach would be to:
1. Transmit on current ATS frequency or 121.5MHz as appropriate, when you normally would during the emergency drill.
2. Transmit on current ATS frequency or 121.5MHz as appropriate, just before hitting the water.
The best of both worlds?
POL