PFD was already mentioned - I'd just add to get one that has a tether. We always have our son in his PFD and tethered when underway. He turned 4 in February and has been on a few trips at the age of 6 months to 1.5 years old. At 1.5 he helped us move our boat from St Lucia to the BVI with stops in Isle de Saints in Guadeloupe and St Kitts before ending in the BVI. Since then he's been on the boat back in the NE in Jamestown RI. We always make sure to bring things to keep him busy - toys, puzzle, books.. whatever they like.

Hoping she knows how to swim, but if she doesn't, get her in swim lessons right away. Very important skill to have in general and even more so when on a boat.