First, this is indeed a simple dive. But losing interest after 10 or 15 minutes is more a function of your attention level than the spring. I 've been to Alexander twice and thoroughly enjoyed it. I have about 175 minutes UW there, and I'm looking forward to returning. It all depends on why you dive.
During my dives I had several highlights. Exploring the crevices is pretty cool. If you take it slow you can get fairly much inside these holes and explore the interior. This isn't a cave dive, but there are some beautiful formations if you slow down and look. I also spent the better part of an hour watching turtles from UW. Counted at least 6 different types of turtles, and you can actually see how they spread their toes when they swim - but only if you look. On another dive, I was playing in the weedlines and just stopped and slowed my breathing. Within minutes there were hoards of crappie & bluegills coming to explore me. Got so close to look into my mask that I could see the red marking inside their throats, as well as the 3 mini teeth tucked just inside their lips. Back in the "deep" areas there also a fair amount of digging thru the rubble to find treasures that the boil kicks out.
Hey I like Alexander, but you have to be ready to do some slow exploration, and to look at things twice. This is a lovely example of a FL ecosystem that most folks here never see.