You can swim anytime from the beaches of OC. The water is still warm in September (I've had my kids swim as late in the year as Thanksgiving!) and you can have a great swim.
Be aware that 26 September is the last day beach patrol staff are guarding the beach. There are fewer of them than in July and August, and they may only be at the south end of town. After this day the guards (and OC police) patrol the beaches on ATVs and SUVs, but the coverage is pretty infrequent.
Outdoor pools at some places may still be open this weekend, but most will be closed. Check with the place where you plan to stay to find out. Several places now also have indoor pools including the Flamingo, Holiday Inn, The Grand, and the Hilton.
Most restaurants will still be open this weekend.Check out some of the other postings for great suggestions, a few include Harborside, Captain's Table, Seacrets, Dead Freddies. If fish Tales is still open they are also an excellent choice.
In terms of deals on the BW, you will see some. As it gets deeper into the off season you will see more and better deals as places liquidate this year's merchandise.
Another great deal this weekend is Sunfest - lots of vendors, free entertainment and free concerts. Check out this link for more information:
http://ococean.com/events/sunfest Have a great trip. I'm going the first weekend in October and can't wait to get back down there!