Hi Samantha,
My family loves water parks, so we always try to dive into some fun at either
Disney's Blizzard Beach or
Disney's Typhoon Lagoon during our vacations. One thing we love about both water parks is that the whole family can have a great time since there are activities for all ages, but that variety makes it a bit difficult to give you specific water depths. In general, it's been my experience that water can range from ankle-deep to 12-feet deep.
The shallowest water is generally found in the play areas dedicated to younger children 48 inches or shorter, Tike's Peak at Blizzard Beach and Ketchakiddee Creek at Typhoon Lagoon. You'll also find lazy rivers at both water parks, Cross Country Creek at Blizzard Beach and Castaway Creek at Typhoon Lagoon. From what I could find, it looks like Blizzard Beach's lazy river is about 2-3 feet deep and Typhoon Lagoon's about 3-4 feet deep. In terms of the deepest water, at Typhoon Lagoon Shark Reef is around 12-feet deep, and while I don't think the Surf Pool is quite that deep, its waves can be 6-feet high, so it's best for strong swimmers. Blizzard Beach also has a second play area for kids, Ski Patrol Training Camp, with 8-foot deep waters.
Life jackets are available at both water parks free of charge, as well. If you're trying to decide between the two water parks, there's a great article on the wdwprepschool.com site - just search for "Battle of the Disney World Water Parks."
I hope that helps with your vacation planning, Samantha, but please let me know if you have any other questions.
Beth