As others have said, you can checkin early (possibly as early as 6 am). Once you are a guest of the hotel, you may use all of the facilities of the hotel (SAB once it opens at 10 am). Your park tickets and dining credits if you have them will be valid to use once you have checked in, even if your room isn't ready yet.
If you would like to learn more about the Beach and Yacht Club resort, check out the FAQ forum by clicking on the link in my signature. There's lots of pictures and info on the first page of this thread.
We often do this on purpose. Take the super early morning flight and get to YC/BC by 10am. Hit the pool, grab some snacks and drinks. It is a great way to ease into Disney, particularly if you are not planning to go to a park that morning.
There are some great areas to rest or just soak up the resort -- SAB of course, but also both lobbies, the solorium at BC, two quiet pools (three if you count BCV) stroll around the Boardwalk. Or play some mini golf at the Swan and Dolphin. Have a great trip!!.
The PP has gotten good info on this thread. I just wanted to point out that the same applies to the other end of the reservation - you're still allowed to use all the resort amenities through the end of the day you check out.