Mike is right...as usual. I've done 2 Alaska cruises, one in May and one in Sept. On the Sept cruise, we ran into some very cold rain. But, I had a good poncho, and just added some layers under that and I was fine. I did need a couple of long sleeved tops, which I usually don't need. Re. the shoes, I would leave the flip flops at home and go for a good pair of sneakers instead. Even for going to the pool and back. But, then again, my feet get cold quickly.
Alaska is spectacular. Don't worry too much about the weather, because whatever you expect won't happen. It just changes a lot sometimes. Have a great time!!!