I live in the area, 15 min. from Inks. I would say that any of the Highland Lakes, lakes Buchanan, Inks, Marble Falls, and LBJ are all not known for great diving. Now, back in August 2007, me and a bud had been out at Inks and we went to Devils Water Hole to spearfish, and the vis was at the very best 8-10', maybe a bit less. We were also just free diving then, and didn't get below 10-15 feet deep. Also, this will be quite a hike with full dive gear. I haven't been down to Devils Water Hole but maybe once since, and it was very murky. Also, I check the water around their fishing piers to look at the water and it's been stained every time. I normaly check it out every few days, though I hadn't been out there in a week or two. I also have never dove Inks Lake on full scuba, so it may clear up once you get below 10-20'. I wished I knew ! Check here on the Swamp Diver's Dive Condition Reports thread for updates - hopefully someone will dive it one day soon and will let us know.
Also, Devils Water Hole is an area of Inks Lake State Parks, accessible through the park, and is at the far north end and in a cove of the lake. There is a long granite gravel trail leading to a large rock and ending at the rock and the water. If you climb over the edge of the water, onto the left side of the rock, and around the other side of the rock, there is a flat area facing the water where we spearfished and free dove. This is where it was clearest, and will be the most free of people swimming. It also makes for easy entry, through exiting here may be difficult, you might have to take off your BC/tank and exit, then pick your gear from the water, and then help your dive buddy out - at least on the rock ledge. If you go straight to an area of shore where actual land meets the water, and not this large rock, exiting will be a normal shore exit.