I dive my BP/W from a kayak and you have to doff/donn it in the water. It is really easy. Here is my procedure.
First, enough air to be barely positive. Slip one arm in up to elbow, slip other arm in and pull both up together. Put reg in mouth, roll forward, catch crotch strap and then roll vertical and cinch it all down. Lastly situate the rest of the rig (necklace, long-hose, etc).
To doff it is just as easy. Again, just enough air to barely be positive. Undo all the parts (necklace, long-hose, etc). Next, unclip the waist and release the crotch strap. You can now simply drop your arms and kick forward out of it or you can reach behind you with both arms, towards the rear clip, and pull it down to your elbows and pull your arms out one at a time.
It is all about practice and procedure. Once you get it down it is really easy. I can doff/donn my rig quicker and people with BCs (not counting the long-hose and necklace).