We have two of these on board, Cal June / Jim Buoy Child's Safety Harness
I have a Mustang Infant PFD if you want it, it yours, we have no need for it anymore.
Mustang Lil' Legends Infant Floater Life Jacket / PFD
In addition, if you can find space, a "pack & play" is great for the boat
Also something like a car seat or seat with harness can be useful such as this