If you flip in rough cold weather (rather than driving like a twat in the bay in August), the odds are a) you won't be able to get to the boat in your wet weather gear and the cold and b) you wouldn't want to go under it even if you could! There'll be crap everywhere and lifelines to catch and trap you, aside from the fact you'll have 1/2 to 1.5 ton of weight bouncing around in the waves to knock you out!
A HH vhf will likely have smashed off you unless tethered. But worth having if you can.
A PLB tethered to you is ideal or at least a phone in a waterproof bag is a good idea if inshore.
Reading the Round Ireland thread of the SR Ribnet member who ejected crossing the Irish Sea is a great read and VERY enlightening! (poly or Willk will find it).
Round ireland 2013