We took our now 16 month old sailing when she was 2 weeks old. Both mother and baby seem much more relaxed on board. We would use the front carrier when the baby was on deck and use the car seat when she was down below unattended. I wedge the car seat in the quarter berth so it was easy to get to, well anchored in place, and safe for the baby. My daughter rode out some pretty rough weather in there.
As said before infants will sleep most of the time. We found that changing an infant on board was not an issue... except for storage of dirty diapers. That one is rough on an overnight trip in the summer.
My wife said the only place she was comfortable when she was at 40 weeks was on the boat. She fell asleep as soon as the sails went up.
I can't wait to get the boat going again this season and to get the family back out.