We are traveling to the cinque terre for a day trip with 19 family members from our house rental an hour inland. We are 10 kids and 9 adults. We would like to be on the ferry or boat, so that we can explore the different towns. Is there a recommendation on how to best do this? I looked into private boat rentals and they are really expensive to the tune of 3600 euro for the day.
The public ferries hold over 300 people but still get quite crowded. You can use them, but it will be hard to stick together during boarding and debarking down the narrow gangplank. The same is true for trains in the region. Make sure that everyone knows the plan and probably that parents are responsible for their own children, or similar strategy.
I would also suggest you have a plan in place (and review that plan often!) for where to meet when/if any of you do get separated from the group. We did/do this with our kids and grandkids whenever we travel by public transportation.
Depending on when you are going, the ferries may or may not be running. They do not run during inclement weather. They will be just as crowded as the trains. The towns are literally a few minutes apart by train, so that may be your best bet. I suggest since you have such a large group, arrange for buddy systems so everyone has a partner, or two, so no one gets left behind!