I bought the water bladder thing independently of the PFD. It's n the fridge at my work so I can't check, but I would doubt it is more than 1 litre.
The pocket would fit a much bigger one.
I found it almost impossible to get a decent drink out of it through the tube / straw thing anyway.
I carry water bottles on my boards for hydration - I have fitted Stic It I think pads and put on bike holders to carry a water bottle.
Helmy downwinded my V1 at Merimbula, and lost the water bottle at Pambula River mouth - first wave took it out.
I have seen JM and DC prone boards with water bottles mounted I think in a fin box on the deck - can you add to that Phil?
Once it is hot I wear a very loose top for sun protection, but still end up drenched in sweat.
I'm not racing anyone so I can cool off easily though
Water bottle mounts