I've rushed on here as I'm a little in the know due to my preschool teaching......so here goes
First off C.P is infectious 10 days before the spots come out so in theroy anyone that has had contact with Kacee and not had them can get them. They are contagious until the LAST spot has scabbed over, then it's ok for them to mix with others. Piriton is good for relieving the itch, as it MEDISED which is paracetamol based (so no mixing with any other type of paracetamol suspension, but Nurofen is fine) and also contains an ingredient to help them sleep and is often recommended for C.P.
As for your little one, he is very young and while it's not uncommon for small babies to get it, rarely those under six months actually do. Exposure can help build up a natural immunity and children in a family with C.P very rarely develop shingles in latter life too.
My daughter had C.P and only came out in four spots! I breast fed her for the first year which helped

But when Kacee has not had a spot appear for two days on the trot then it is showing signs of clearing up. As long as all the spots showing appear to have scabs then you can't infect anyone. It's usually around ten days after the first spot has surfaced, but eowyn was "clear" after 5.
Pregnant women at certain stages of pregnancy can develop, shingles but all pregnant women will tell you they stay away from anything remotely looking like a "disease" lol.
So fingers crossed you will be ok, I very much doubt that anyone will check to see if your child has spots when you board the ferry, but as you say, you just want peace of mind.
I wouldn't worry as it's a common childhood thing and I bet you there are a few more kids on the ferry with far nastier germs (lol)
Lisa x
And as Cyrano pointed out, siblings will show signs around two-three weeks later. My daughter however took six weeks to show up!!