I contacted them, and here was their response.
Justin,
I have included a link below that contains our current baggage policy and our revised baggage policy effective 6/10/08 pertaining to SCUBA gear.
Baggage Chart - Frontier Airlines
The link states the following:
SCUBAOne bag which may include fins, mask, knife, snorkel, regulator, safety vest, spear gun, pressure gauge, tank harness, empty scuba tank (with the regulator valve completely removed).
NOTE: Knife and spear gun must be checkedFree - packed within or in place of one FBA
Effective June 10, 2008, a fee of $100 will be charged to check scuba items.Checked: Yes Carry-on: YesYes
That is pretty much what I posted in post #4 from the same source. At least they are being consistent. I'm still not sure if that is one-way or round trip. But they would have to be a whole lot cheaper (or I use my freq flyer miles) before I'd even consider paying $100 extra.