There are quite a few places to dive within 3-5 hours of STL. You mentioned Mermet Springs and Bonne Terre Mine. I agree with Steve C, for a close to home, easy dive... Dive Stop is a fine locale. I enjoy diving Table Rock Lake, Quail Run Quarry in Rolla, MO (once in awhile) and Mermet Springs. Bullshoals, Norfork and Stockton lakes are common to dive. Haigh Quarry in Kankakee, IL has a very nice quarry. Lake Michigan is home to some awesome shipwreck diving.
I enjoy these dive sites year round and enjoy local diving immensely. To me, this is what diving is all about...this is what I refer to a lot of times as "REAL" diving. As many of us only see the beautiful tropical waters of paradise once maybe twice a year. So, unless you're a vacation diver this is our reality. Which in itself is not for everyone.
Mermet Springs with its docks, platforms, various depths and attractions is very nice. The amenities and pavilions with tables to gear up on are great! Dive Stop is close to home and is a very convenient dive site. Table Rock Lake...I grew up camping around the lake, fishing on the lake and now diving in the lake. I am not sure if it's sentimental or it's one of those places where you feel you can truly get away and relax. It's a large lake and is far from the party scene of the Lake of the Ozarks! The Kimberling City bridge is a popular dive site along with Dewey Short (the Dam) along with several others.
I am easy! I just like to dive.