Might be worth looking into D3o armor inserts. My GS racing suit uses D3o inserts and they work very well, the stuff is a gel that stiffens up when impacted - although I think it is more designed for a sharp impact than something more spread out like hitting the water.
Otherwise, I think that Radars X vest seems to provide me a lot of protection when hitting the water. It isn't super thick but is uscg and has more padding than a comp vest.