We have been taking our daughter sailing since she was two weeks old. We would have gone earlier but Mom had a C-section and wanted a little time to heal.
Anyway, at first she was in a boppy carrier (Mini-Me carrier) on her mother or down below in her car seat witch is secured in the quarter berth. Now that she is a year old, we have her in a PFD and let her crawl around the cockpit. She is not able to climb out of the cockpit so we do not have her in a harness yet. That will be next season.
Getting the kid used to the PFD has been a several month battle. We first tried it out in the swimming pool. I have to say I wish my PFD had a built in head rest. It looks quite comfortable. Then we made her wear it for increasing amounts of time while underway until she got used to it. It is hard for her to crawl in the PFD so that is the only time she screams about it. When she is not wearing the PFD and we are underway she is in her car seat down below.