I agree with John above quick call to London info and then switch without delay to D&D.
Most airliners are listening to 121.5 these days so if you can broadcast a radial and distance from a nav aid (your GPS should give you the nearest nav aid or airport) the guys in the airliners can put this into the fix page on the FMC and pinpoint your position , they can't come to your aid but they can pass the info on and they will be able to hear you long after you are out of range of ground stations so can pass on a fix given just before you hit the water.