You've probably read this, but I thought it would be worth posting what Delta's web site says:
Scuba Equipment
One dive bag containing scuba equipment other than an empty dive tank is allowed as checked baggage. An empty dive tank is allowed with a USD150 USD200 fee each way. The maximum outside linear dimensions may exceed 62 linear inches (158 cm), but oversized baggage fees will be applied. Tanks weighing over 70 lbs will be charged the applicable excess weight fee. Tanks exceeding 100 lbs or 115 linear inches are not allowed.
Scuba Equipment Fees
*CAD amount will be charged exit Canada, and EUR amount will be charged exit Europe.
- 150 USD/CAD or 105 EUR* for travel to all regions (excluding Brazil)
- 100 USD for travel to/from Brazil
Scuba equipment that exceeds standard baggage allowance will be assessed the appropriate fee based on destination.
I don't think she was asking about their policy, but rather what they do in practice.
Despite this policy being in effect, I have made multiple trips to Mexico with two suitcases loaded with dive gear and have not been charged any baggage fees (I get a free bag due to my Delta AMEX card, and another due to Sky Miles elite status).