Stainless steel bottles are fine. And food, too including bottled water.
Here's a comprehensive list of prohibited items:
The following are not allowed to be brought into the Disney Theme Parks:
* Items with wheels, such as wagons, skateboards, scooters, inline skates, shoes with built-in wheels, two-wheeled or three-wheeled conveyances, strollers larger than 36" x 52", suitcases, coolers, or backpacks with or without wheels larger than 24" long x 15" wide x 18" high (coolers required for medication may be stored in a locker or at Guest Relations), and any trailer-like object that is pushed or towed by an ECV wheelchair or stroller
* Alcoholic beverages
* Weapons of any kind
* Folding chairs
* Glass containers (excluding baby food jars and perfume bottles)
* Pets (unless they are service animals)
* In Disney's Animal Kingdom Theme Park (for the safety of the wildlife), balloons, straws and drink lids are not permitted. Note: they now have biodegradable paper straws.
1971 (age 15) camped off-site MK was new!
1974 off-site (Senior Trip)
May 2002 AS-Sports, with DW & kids
May 2004 Pop Century
Feb 2005 Wilderness Lodge Wow!
Oct 2006 Pop Century
Oct 2008 Camped at Fort Wilderness
Feb 2010 Disney Cruise - the Wonder