Rule-of-thumb is that if you're going to the hotel for an activity that you have to pay for, you're more than welcome to (Disney won't turn down money). This includes restaurants, bars, boat rentals, fitness centers, bike rentals and a bunch of stuff they offer at Fort Wilderness. Walking around and looking at the hotel is OK, too. (Because it might spur you to want to stay there in the future)
Resort features that are "free" like pools, volleyball, tennis courts, etc enters a *very* grey area......
-Rob