We took Jacob on his first cruise when he was 20 months old. Just me, Vanessa, and Jacob. Looking back at it, it is better to do this with others in your family coming along who are willing to share the baby duty - as happend with Chef Ken's cruise.
I do recall the wonderful waiter we had (this was on the Celebrity Century, when it was still a Chandris owned line) who would pick Jacob up and carry him around the dining room whenever he started to get a bit whiny (the baby, not the waiter). This always stopped the crying and we'd get back a laughing little boy. That waiter got an impressive tip from us, and he well earned it.