I have friends with a sailboat who wanted to take their <1yr infant on their sailboat with them, and so consulted the (Canadian) coastguard about how best to protect her.
The good news is that very young children (< 2 yrs, I think) don't count as passengers from a legal point of view; so there are no legal requirements to fulfil, just what you need to do to be as safe as you feel necessary (again, that may be specific to Canada)
Their approach to this was to get a standard carrying seat and strap on to it enough flotation material (mainly polystyrene foam, I believe) until the seat floated upright in water supporting an equivalent weight to their daughter. They then took her sailing on the Great Lakes. The seat already came with straps to hold her in place, and she could be left in the seat (below decks or otherwise in a safe space) during docking and other maneuvers that required two people. Actually I think she was left in the seat pretty much all the time they were under way.
The daughter is alive and well and studying marine biology, so it must have worked out.