We took our little one out at 5 weeks, and while we had a PFD aboard, we never put him in it. While underway, he was in the car seat, which was strapped down to the boat. Basically, he would only get wet if the boat sank. When moving from ship to shore, he was strapped to momma in a frontpack. If she went overboard while boarding the dinghy, the plan was for her to start doing backstroke.
To more directly answer your question here is an article that might help. I like that there is at least one company out there that designs these things from scratch instead of just making a "smaller adult PFD".
Outisde magazine Infant PFD
The PFD from the article:http://www.salusmarine.com/product/bijoux.html
Video of mom trying
to drown her infant while wearing the above recommended PFD. Spoiler alert (she can't seem to drown the baby):http://www.salusmarine.com/press/bijoux1.html