Not Celebrity, but we just cruised with an 11 month old. Totally doable. I dressed him mostly in rompers and a few short/t-shirt combos. Only 1 or 2 outfits per day. My kids were never spitters, though I hear that gets better around 6 months. We got him a plaid collared romper for formal night. It was much more fun dressing up my 3 year old because she loves to dress up.
Get baby used to sleeping in a pack n play. Mine ended up co-sleeping most of the time. He would NOT sleep in the hotel pnps at all. Carnival has actual mini cribs and he did sleep in that for 3-5 hours per night before joining us in bed.
If baby is crawling, but not walking (very likely scenario at 8 months) try to find somewhere everyday to let him crawl around. My son got super restless on days when he was in a baby carrier or stroller a lot.
Eat in the main dining room as much as you can. The buffet is kind of a pain with a baby (and toddler in our case). We had to have DH find a table with the kids, then I would go get their food and drop it off. Then I would go get my food,and when I got back DH would take his turn getting food. Better to just be waited on in the MDR 🙂