Although I admit that Molokini isn't my favorite area to dive in Maui, I've rarely been disappointed diving there. What site were you diving that you dropped to 100' and stayed there?
After 80+ dives in the crater, I can't think of one site where staying at 100' would be considered "normal". One of the beauties of Molokini is that dives are typically multi-level profiles, allowing you to go deep if you want (like Shark Condos at 130'), then work your way up the wall and still get a 50-60 minute dive. I tend to not go deeper than about 80' at Molokini, keeping most of my dive between 40-50' where most of the critters are.
Like you, I find the S. Maui dive sites to be more interesting and better than Molokini overall, but at the end of the day, as long as I'm wet, breathing compressed air, and get back on the boat safely, I'm a happy camper!
The promo information on Mike Severn's states that they provide marine biology/ecology dive leaders, marine critter information and short turn around dive charters. It is very clear that they zip out early in the morning and return by 10:00 or 12:00 before the other dive/snorkel cattle boats are around.
If you don't like a little ocean education, don't sign up with a charter that prides it self in being the only boat that has real ecologist/biologists on board.
Most of the Molokini chartersI have been on, with the exception of a back wall dive, do one dive on Molokini and then move to a shallow shore location for the second dive. The last charter I was on was a deep Molokini dive (the garden eels are at 100 fsw) followed by a dive at the Tanks off Makenna (SP?). There was a frog fish and a two really cool Harliquin shrimp at that location.