Re: How to secure a baby
Consider some strips of wide adhesive velcro, and using them to add padding (yoga mat, camping pad, exercise mat) in the area where you'll be stowing the baby "just in case".
Vecro, btw, is often underappreciated. A couple of large patches, allowed to bond overnight before being used, can help secure a LOT of weight.
"The baby MUST be wearing a PFD on board. There is no way a baby can sit in a car seat with a PFD," I'd suggest you haven't spent much time in the water with PFDs. Very few of them will keep anyone's face or mouth out of the water, and those that will are so bulky (typically Type1) that they are shunned. I have doubts that any "baby PFD" would do anything except, sadly, make body recovery faster. (Which is all any PFD will do for anyone in icy waters.) A car seat might indeed work BETTER since the seat will probably orient the baby's head and face above the water, and the seat itself may have net bouyancy, especially if it is rotomolded plastic and hollow, or uses closed cell foam. You can't begin to guess without putting the specific seat in the water.
To abuse a phrase, there's no need to go overboard with safety.