HM gives good advice -
take on board rather than
buy on board. Even if money isn't tight, it's always better to spend it on things you choose, rather than stumping up for overpriced goods you didn't think you'd need.
Cabins tend to be small. There won't be much distance between the bathroom and the bed. Take scented candles if you get my drift, it will help maintain a honeymoon atmosphere.
Don't be embarrassed to take some snacks, or buy them in port. There will be some evenings you want to just chill and chat in your cabin, and it's not like in a hotel where you can pop out to buy olives, crisps (chips to you), pretzels etc.
BOOKS! I'd probably read while I was queuing to get into hell, but even slow readers seem to get into books more if there's no land in sight. If you don't usually read much then just take 2 and figure you'll swap once you get to know people.
Lip balm (and cold sore lotion if you're prone) For both of you. Sun and sea spray will affect you much more than a normal beach holiday - look after those kissing cherries! My ex had lips like a condemned building after about 3 days. Luckily I had an ungirly selection for him to choose from.
Read up on what's available/ scheduled on board. Some cruises have themed evenings with prizes for best dressed etc, or special sports events. It being your honeymoon you might not be interested
but it's worth being aware before you go.
Finally - enjoy! What a wonderful honeymoon, have the time of your life.