Since my last attempt at replying timed out, I will have to give the short version.
Shore Diving:
Mansfield Park area. You have dove the dive area already so I assume you have seen the wrecks. You can also dive the wall or dive the river channel for deep diving. You can also dive the cove in the Day Use area. There are some grottos off the nearside point about an 8-minute u/w swim along the wall. Be advised that you will be under boat lanes. There are also grottos at the point on the farside of the cove. You can also dive around the corner from the farside point. You can dive in the cove for that matter. Depth of the cove get deeper towards the mouth of the cove.
Bob Wentz Windy Point Park. A Travis County park for day use with restrooms and picnic areas. The wall hear runs north-south and there are grottos here also. If you swim south along the wall you will find even more grottos, but you can end up under boat lanes. Good place for shallowing diving also.
Windy Point owned by Richard Barstow. A continuation of the diving area from Bob Wentz park. Richard has placed platforms for classes, statues and wrecks. There are ladders to get into the water. Richard runs a good operation. Camping, showers, restrooms (Chemcans).
Boat Diving:
Starnes Island. Probably the best place for diving on Lake Travis. Walls and usually pretty good visability. Cool place to camp too, IMHO.
Anchor Point. This is what I have known it as. This is just to the north of Starnes Island. A shop I taught for used to do their open water dives off a boat there. The north side of the point is a gradual slope and the other side is a mini-wall.
Oasis Restaurant. Not under the table, heh, heh, but below the restaurant! Good diving in the coves. Found several anchors there.
For grotto lurking, all the grottos I have found are at 600 feet MSL. So subtract 600 from the lake level and you have the depth of the grottos.
These are a few of my favorites. Have fun!