There is nothing made--except maybe custom, that size. The smallest I know of is for Max. 59" 99cm - 150cm 44lbs - 110lbs. by Mustang. I have taken babies as young as a month boating, on all types of boats--from open dinghies, to large cruising boats. In this case, you are the ideal buyer of C pup's off shore survival raft, complete! There are a lot of issues with survival of in infant, as well as hypothermia--which the Immersion suit is designed to help prevent. keeping a baby who is free in the water from drowning, (unless the infant has been taught to swim--and then the immersion suit becomes a hinderance). (My children were swimming by age 6 months, thanks to Greta Anderson's program--but not in the cold water of AK. )
A papoose type of "suit" made of thick neoprene, with a zipper and neck seal for the size of the child. (Neck grows much more slowly in diameter than the body in length). (all self made with material at home) would be another solution.)