SPI has some great dives reasonably close to shore. Our two most popular dives are the Texas Clipper and the Rigs (the Iron Reefs). There are a bunch of wrecks sunk around SPI too that used to be our bread and butter, but because of the popularity of the Clipper and easy access to the Rigs, they have become our mainstays. That being said, I'm going on my third year diving the Texas Clipper and have yet to see all that she has to offer.
For $300/diver you can do two days of 2 tank diving. Lodging is fairly reasonable and a couple of Hotels offer diver's discounts (namely the Howard Johnson and the Suites at Sunchase). We repackage hotel slots to diver's for $149/night, but you may be able to get a better price by calling the hotel directly. That being said, you and a buddy could share a room and get in 4 dives a piece for under $450/ each.
American Diving has been diving the Gulf waters off of SPI for 25 years. We have the most experienced crew in the area and are the Island's only full service dive store.
South Padre has much to offer during surface intervals or for non-divers as well. Including Dolphin Watches, Eco-tours, a renowned birding center, Sea Turtle, Inc turtle rescue, Schlitterbahn waterpark, as well as world-class kiteboarding and some decent surf in the winter months. I hope folks take the chance to come dive with us this summer. Also hope that clean-up and recovery happens quickly so that our fellow dive ops in the gulf are able to resume normal operations.
Safe diving, Bryan