I have sailed with many infants and they have snoozed safe in hammocks. The only problem with car seats strapped below or above is that if you do any cruising and happen to run into bumpy water or a little blow, the baby is literally thrown around in the constraints of the seat instead of the gimballed rocking you would get from a hammock. A baby, in my experience, cannot roll over in a hammock. Just do not place something like the blanket up around his or her head to avoid the risk of the blanket working it''s way across the baby''s face and it smothering. I have met cruisers on monohulls and on cats whose 2, 3, and 4 year olds sleep in hammocks.
There is something so endearingly peaceful about a baby asleep at sea, makes me smile just thinking about it, whether it''s an infant in a hammock aboard a 32 footer or a 2 year old in his playpen on a 50 foot cat.
Best of luck to you,
MaryBeth