We started Lilly at 7 weeks on the boat. We had a 40 ft cat. Pretty simple with her. Some tricks:
1. Bring diapers as you can't be assured you will find the size the need
2. A cockpit cooler is a great bathtub and play zone
3. Odds are the child will be exhausted and sleep through dinner, so we would just bring a setup that we could create a pallet for her to sleep
4. There were some buzz kill moments, but I think that was as she got a bit older
5. Easy to rig a swing with the boson chair from the clew of the jib sheets to a lot of fun.

The trip won't be as much fun, but we were not too concerned about safety, we were the horrible parents that didn't even have life jacket requirement, but our cat was pretty safe boat.