On our last cruise, Serenade of the Seas, we bought 2 six packs at the pier store and just carried it on board in the plastic bag they sold it to us in. We mixed drinks in our room and put them in travel mugs. We brought those out to the pool. Never had a problem.
Yes there is the "pay fridge" which is maintained by the cabin steward, But they are small / limited and thankfully no automatic reception billing sections on take out . The steward checks usage, bills your cabin and replaces content daily
So I would recommend
Tell your cabin steward to empty the fridge of overpriced water, soft drinks, beer etc as you do not want them.
Then bring your own on board, bottled soft drinks, water are not a problem, but remember everything goes through some sort of metal detector when you come back onboard (Airline conveyor style or personal walk through), so metal will flag. And they will question and I've seen conviscation of spirits and canned beer, to be reclaimed at the end of the cruise.
Yes they will do this, as thats how they make money, your bar tab
Glass or plastic is better, but for safety buy only brand names in ports visited.
We got our steaward to empty the fridge, so we could stock with our drinks and bottled water as I will not drink warm recycled water from the ship or pay their inflated prices for bottled
We just got back from a cruise on the Adventure of the Seas. We brought a 12 pack of coke and bottle of rum in our carrying on bag and had no problems. We also brought a case of beer in a suitcase (bought in San Juan) and also had no problems. It was very nice to come back from an excursion and have a cold beer waiting for my husband. Hope this helped.