Bottled water is available but quite expensive.
You can always get water at the buffet or from the tap in your cabin. Tap water is distilled right on the ship and tastes quite good and is probably purer than any bottled water you can buy.
We usually bring a couple of bottles (different sizes) from home and just refill them from the tap or water station in the buffet.
Bottled water is available in your stateroom each day. It will be replaced as you use it. The cost was approx. $4.00 for a 1.5 liter bottle. You may also purchase ice cold bottled water as you leave the ship on shore excursions. They have drink holders with a strap to make it easy to carry with you.
I've sailed princess several times. We've had no bad experience with the tap water. Mattox is correct, the ship uses reverse osmosis filters and the water is as clean or better than most bottled water. I've only gotten sick one time and I'm pretty sure it was not from the water on board, but from food eaten ashore.
If you want to be certain even though the above remarks have been stated, buy big bottles at your local grocery and pack them. When you go on excursions or into port you may have the opportunitiy to buy large bottles of water.
I tend to be a bit **** about the source from which water comes. My city tells me it is safe but we filter it and it tastes better. So if the ship says its safe I might take the same method to sea with me.
I imagine that they get sea water because they have huge desalination plants aboard the vessels. I can't comment authoritatively on the filtering process. My attitude, (right or wrong) tends to follow the idea of drinking the water from the bathrooms on airliners. I simply don't. (and I am not referring to just the "blue water!" either)
Actually, to remove all nervousness regarding water on board, I chose to enjoy highly distilled and peat-fired grain beverages. oh, and of course the bottled water. Jameson Irish Whisky! I was such a lover of Scotch until I tried it. As I ramble on, try a drink that I call "Warm Summer Day" Jameson and Sierra Mist or 7.
to ease your mind about tha tap water on board, it is both distilled AND filtered; you can't get it any purer than that. I doubt if rainwater is that clean.
However, I also prefer other distilled beverages, particularly those flavored with juniper berries and blue agave cactus.
A cruise just isn't a cruise for me without my evening martini or two...and this is a real martini made with gin. I've never accepted the bastardization of martinis with vodka, chocolate liquer, etc.
Although ships have distilling plants they don't totally rely on them. When they can they will fill their tanks with potable water from a pier and this is also filtered and tested. This is what we did in the Navy too - we'd buy water from a port whenever we could since it simply is a faster way to top off your potable water tanks.