Hi Becky and welcome to SoCalHiker!
The Trans-Catalina Trail is beautiful, and feels like a world away from the rest of SoCal. It was my girlfriend’s first backpacking trip when we were training for the JMT. In some ways it’s a great first trip.
Note that the route has changed and our guide has not yet been updated to reflect the change. The changes make it easier, and reduce the total mileage hiked. Previously the trail “ended” at Starlight Beach — a full 9 miles from Two Harbors — and that “extra” milage back to town wasn’t considered part of the trail. Now it actually ends back at Two Harbors, so the mileage for the TCT is an accurate reflection of the miles you’ll hike.
As for water, no need to filter or treat. And yes, water is part of the deal when you book your campsites. A few of the campsites have water on tap, and those that do not, a locker is provided where you’ll pick up your supply of bottled water. The system works pretty well.
Be prepared for cold, damp nights and hot, sunny days (unless it’s raining). And have a blast!