Try to get there and check in by 11:20 or so. There are 12 waterside tables, and currently there are less than 12 11:30 reservations, so if you make your desire for waterside known to the host when you check in you should get it.
As to an earlier comment, you cannot reserve waterside. You must check in with your entire party, and request waterside. The host will get an estimated wait time from the lead inside, and you can then decide whether you want to wait or not. It is always worth asking, because sometimes the response is "right away", although sometimes it is 20-40 minutes, often longer for parties larger than 4.