It's nice to hear from you, and I'm glad to hear you'll have an opportunity to try out our lake. Visibility this past weekend while I was teaching a class was almost 30ft, water temps 71 surface, 68 at 70ft.
Now, I'm not sure what you're referring to when you mentioned a "dive park". We really don't have anything like that. There is what we call Scuba Cove, which is just to the right of Diablo East Marina. However, it's not what I'd call easily accessible. Once you turn off of Hwy 90 at the Diablo East road, you'll go maybe a quarter of a mile and see a very worn Park Service sing for fishing/swimming on the right. This is a rather lengthy winding dirt road that takes you by a number of picnic areas. The two most frequented spots are the cliffs, followed by the cove. The cove parking circle is as high as the cliffs and you have a pretty steep walk down to the shore...not for the weak, that's for certain, particularly carrying scuba gear.
No one in Del Rio does Nitrox, sad to say. There are only three places to get regular air fills in Del Rio. The primary is at Amistad Scuba, also known as Amistad Propane. They are on the right side of Hwy 90 on your way out of town (just across from the Exxon station). Ignore the two story building on the left just before that--it used to be Amistad Oupost, but they went out of business last year. Anyway, Amistad Scuba, as you noted, closes at noon or 1:00 on Saturday. If you call ahead and coordinate they'll often accommodate divers who need a fill. The other two ways to get fills would be to contact myself or the other PADI instructor in the area, Richard Urban. He lives out by Rough Canyon. We each have our own Max-Air 90 portable compressors that we purchased mostly for the convenience of filling tanks while we're teaching. Neither of us are really advertising air fills, but if I'm around I can usually be persuaded.
Regarding places to dive without a boat, you have the cove, you have Governor's Landing (take the turnoff for the International Dam and you'll see the road to Gov's Landing), and the Old 277 bridge (also used as a boat ramp since most of it is submerged, so be careful). Really, just about anywhere you can reach the water from a road there's a place you can dive. Speaking for myself, I prefer wall diving, so anywhere that I can access a submerged cliff, I'm happy.
Although I'm teaching a class this week, it looks like Saturday and Sunday are mostly free. So, depending on what time you want to dive, I'm up for it and I'm guessing ScubaBob, our club VP, may also be available. We can take you on "the tour", which amounts to visiting a couple of simple sunken boats. Then maybe head to the cliffs and drop on the wall a bit.