Re: Seatbelts!
After reading this, the first thing to that comes to mind is like mentioned above, is rescue in the event of sinking or capsize. A rescue swimmer isn't going to recognize the seat belt or how to release it. I like the Velcro idea but that too can be a problem to release if you don't know it's there.
Maybe a molded seat with a padded drop bar of sorts, something like those on an amusement ride. It would provide a forward rest, keep her seated, but not be so restrictive that someone couldn't pull her out without lifting the bar.
I have seen seat belts in some rigid inflatables so they do make a marine rated seat belt for a boat. Be careful if using a race car intended seat, many are not intended to be used for long periods of time and can be rather uncomfortable in rough conditions. The padding may also soak up water like a sponge. I do know that those blow molded seats will fill up with water in the rain, so their most likely not suited for boat use.
Something as simple as a good captains chair may work too, with the addition of a simple belt or bar, plus a foot rest if needed, they can be really comfortable.
I did see one boat a number of years ago in which the rear seat had been fitted with sort of a wide belt of nylon net type of restraint to hold a double amputee safely seated. It basically wrapped around his torso and was Velcro latched to the seat next to him.
Another option may be to just attach Velcro directly to the life jacket and to the seat, it would hold a person securely upright but they would be able to pull free in the event of an emergency.
A buddy also had a child seat for his boat which had two quick release straps which clipped directly to the bottom of each side of the life vest. It used those plastic clip on buckles as on a normal life vest but had two additional straps for clipping into the boat. There were two straps bolted to the under side of the seat to attach those to. It was a basically a small molded plastic boat seat on a pedestal. I'm not sure if it was factory or something custom made.
Another thought, most seat belts are built to be corrosion resistant to some extent. One thing to look at would be racing or amusement seat belts which are intended for all weather use. Here's one source that comes to mind:
http://wescoperformance.stores.yahoo.net/industrial-vehicles.html
(Their marine seat belts are the type that flip open with on hand, they are easy to release and the buckles are made from Stainless steel.