What activities/environment are you looking to protect your 1 year old for? No judgement, sarcasm, or being smarty, just brainstorming with you here.
I imagine the 1 YO will always be in your arms in the water, right? If just splashing in fountains on board, you don't need anything flotation-wise.
If in the pool on board, Disney's smallest life vests may be infant sized?
If you are going on an excursion on a smaller boat, you can purchase infant sized (coast guard approved) life vests and travel with them-- or first confirm with the excursion company what they provide on board.
If on an open ocean beach on an island, same thing-- the 1 YO may not be in the water, and definitely not alone?
I understand wanting to be as safe as possible (that's my style) but also be cautious of over packing unnecessarily (which I am afraid of doing myself on a cruise!).