You can ask for a travel cot and baby bath for your cabin. I did this for my sister when we sailed with her and her toddler. You could also consider taking a small blow up paddling pool to put in the shower, we do this for my 2yr old as he hates the shower.
At 15 months they won't be eating baby food/mush still. Plenty of choice in the buffet for toddler friendly finger food, soups, pasta etc. You also won't need to sterilise at that age, but you could take milton tablets if you really want to.