Yes. Not only can you go swimming, you can stow your bags, use a locker, use the locker room - which has showers. (I'm pretty sure no hair dryers though) You can also check -in, they will send you a text when the room is ready. Further, thre's a chance your room will be ready. Further, if you have a package deal- i.e. meals or park tickets- you could head to a park as soon as you get your KTTW card (room key, ticket, meals, charging- all in one card), and you can also start using dining credits right away.
the only thing you won'tbe able to get right away is any luggage tha you tagged with the yellow ME tags. Those won't arrive until late afernoon. So if you want to go swimming, you'll probably have at least one carry one bag (Or maybe wear swim suits under yor clothes?).
Also, AKL has activites all day long. In my opinion, it has the best resort activitiesof any WDw resort.