I am about ready to have a heart attack at all these suggestions of using a car seat on a boat. You might as well tie your baby to the boat anchor.
The baby MUST be wearing a PFD on board. There is no way a baby can sit in a car seat with a PFD, and even if they did, the PFD wouldn't keep the car seat from sinking to the bottom of the water.
Please don't ever ever ever ever ever put your baby in a car seat on a boat. I don't know much about sailing, but I know that much. If you can't find a way to safely transport the baby with a PFD and not
strapped to a boat anchor, don't take the baby on board.
I've only taken my babies on boats a couple of times (never sailing, once in a canoe and once on some kind of motorboat thing in Hawaii), both times they wore their own PFDs and sat on my lap. That's not practical of course for being out all day long or overnight, but it works for a couple of hours.
You might try hanging something like this in your boat: http://us.kanoe.com/
- if you can find a place to hang it where it won't bang against stuff as the boat moves.