After a recent swim (capsizing a sailing dinghy as it happens), I have been considering looking for a BA with a crotch strap like they do with kiddies ones.
Gentlemen with a certain build/bodyshape (perhaps one that makes them better suited to an open Canadian than a kayak, if only for the picnic carrying capacity) probably find, as I do, that they ride up a lot when relied upon for flotation. It makes a bottom strap essential in any case.
By the time I'd swum round the end of a laser 16 in a force 5 wind then half climbed/been half dragged up onto the keel by the harness, it was putting so much pressure on my diaphragm I was having serious difficulty breathing properly.
What you really need to do though is try a load on and see what "sticks" best. Consider upping the bouyancy level if you're considering white water, aerated water does not provide anywhere near as much upthrust.
I'm currently using a palm FX which works pretty well, it's the best I've found so far for my body shape (which is of a big guy with a physical job but who likes more beer and food than is good for him). It has a front pocket and a rescue harness. It's an over the head fitment which some may not like. It has a central strap and a bottom strap to help stop it riding up. It is a bit bulky out front which can impede re-entry if it hooks up on the gunwale.
I'm pretty sure my XL one has 75KN of bouyancy and even that doesn't seem quite enough for me in white water (I'm quite sinky in the water). Many BAs are 50 or 55KN.
But yes, try them on. If you have a paddle sports convention you can go to, that's a good place. I would have tried on up to 20 different BAs before I settled on this one.
"I'm not getting in a boat which is DESIGNED to go upside down."