Hi chrichianti and welcome to Cruise-Chat!
Taking Serenade of the Seas to Alaska will be a wonderful trip, and a great first cruise.
You can't get a balcony cabin too low on the ship, as I believe they start on Deck 7. Some of the rooms at the very back of the ship have larger balconies. Since you'll be sailing on smooth waters in Alaska, taking a room at the back won't present a problem with motion. I have not sailed on Serenade, so I hope someone who has done so will give you more complete advice.