During the off peak times, crowds are very light in the morning, but then people show up in the afternoon. You can easily take a lap early in the morning and you'll have walk on rides. In the afternoon, I'd say the major attractions will have a 30 minute wait.
It really comes down to whether you want to "feel" like you're a part of the action. Stay onsite, and you can walk back and forth in minutes. And since the pools are heated, you could probably use them. The short walk to the parks and the pools are what set the onsite hotels apart from offsite.
If you stay off site, you go early in the morning, and leave by the afternoon, but you save a ton of money. You'll have fun either way.