We mostly drink tap....we stay at the timeshare and have a full fridge, so we usually order a few bottles of water from a grocery delivery service, then refill with tap. We also drink tap in restaurants. There's an infamous 1.5 liter $9 bottle of water at the poolside concession stands, so we just bring our own to the pool areas most of the time. Word-on-the-street is that Paradise Island has its own desalinization plant, and the water that comes from there is very pure.
Going for 3 nights, I wouldn't bother with any off-resort excursions...too much time away from the resort and totally unnecessary for a brief trip.
My little ones did the Sea Squirts several times (
pics and discussion)...its a nice cute activity. While you are over where this activity takes place, make sure you check out the sea-life lagoons between the Coral and Beach towers...kids can get up close at these, and can help out the sea keepers at feeding times (look at the Atlantean daily activity sheet, and look for interactive seakeepers sanctuary).
Otherwise the kids will love the dolphin Cay activities and the kid pool areas.
Also note that the wavy and rapids river system in Aquaventure does not have any height requirement. The major slides all have a 48 inch height requirement, but smaller ones can ride tubes in the river (with freely available life jackets, if they so choose). I've been riding these rivers with them since my youngest was a month or two shy of her third birthday.