Nick is spot on. The round blue hole you see in all the photographs is the collapsed ceiling of an ancient cave.
The thickness of the ceiling is about 110 feet so on the way down you see only a featureless wall until you get around 110-120 feet deep. Another factor is the tour companies that take you to the Blue Hole will not let you dive a shallower depth because it would mean leaving you unchaperoned during the dive. It's 130 feet or nothing.
I agree with Nick again about Turneffe, especially the North end of Turneffe. Beautiful dives, shallower depths, easier and quicker to get to and cost only 165.usd for the day.