From my experience with cruising with 4 babies:
* Lifeline netting is nice but not a requirement. We used it with our first two
kids but then the second two were on a
new boat and we didn't bother with the netting and we were fine.
* Well fitting life jacket and a harness. We had a
rule that the
kids were harnessed whenever in the
cockpit and it was great for peace of mind. They could fall
overboard but never hit the
water.
*
Inflatable bathtub which doubled as a little pool. This was great because it took so little room.
* A secure place to sleep. When baby is not mobile, this was pretty easy but once baby got mobile, it was tougher. I know some people use a pack and play on board to have a secure place to put the baby.
* Small bins to rotate toys. We had shoebox sized boxes and would rotate them so that the toys were still pretty interesting for a long time.
* Diapers - lots of them. We did cloth diapers and it worked really well but not everyone is up for them.
* A seat top high chair like this:
Fisher-Price SpaceSaver High Chair - Tan Lattice - Fisher-Price - Babies "R" Us They also have space saver ones too.
* A good umbrella stroller
If I can think of more, I'll post more but these are things we found to be great with the kids.