Unless the Nanny is a part of the boats crew anyway, I would leave the Nanny at home and share the experience with the child to make it family trip. There is so much bonding time to be had on the water
. To have a nanny there, would simply steal that time away from you.
We had our kids on our boats almost from day one-almost :-D definitely keep the jacket on as you suggested. The rules with our kids, when they were younger, was that the jacket can only come off below decks. But must be put on before climbing the companionway
We also had little ropes the kids could drag in the water
and go "fishing" as they liked to call it. Of course, it was just a piece of rope
with no hook, but they enjoyed it all the same. We had a printout of the local fish
species and they could refer to it, wondering what big sea monster they were about to catch. You could do the same thing with birds and other animals
A pillow case full of favorite toys was also handy. The rules were, if it could fit in the case, they could take it. But we were conscious of two issues. Electronic toys needing batteries and very small toys (like lego) getting lost
. So, small toys could only be played with in one area and electronic toys needed to be shut off. Its the "Pirate Code - Navy
Rules - Law of the Sea" or whatever fantastic fun name you choose to call it. But never, just the boring old landlubber rules lol.
Enjoy the trip :-D