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
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.