We have taken a car seat on fin keel daysailors (Rhodes 19) as a place to put an infant if a 2nd set of hands was needed in a pinch. HOWEVER, we never strapped our kid into it. He could sit in the seat with his PFD on just fine. And depending on your definition of infant, the straps are for a crash, not to keep the kid from getting out of the seat. Another good option is the seat Cruisingdad shows here:
Originally Posted by Cruisingdad
Not nearly as heavy as a car seat, but a good restraint, with or without the strap on.