Their policy:
"The following sporting equipment will not be counted as a piece of your checked baggage
allowance, and require additional fees. (NOTE: Sun Country does not provide bags or boxes for items listed below. Please call Sun Country Reservations at 1-800-359-6786 or 651-905-2737 for Minneapolis/St. Paul local calls (Mon-Sun, 5am-Midnight Central Time), if you plan to bring any of the following items on you on your trip.)
* Scuba-Diving Equipment: One item of scuba diving equipment consists of: One (1) regulator, one (1) pressure gauge, one (1) mask, two (2) fins, one (1) snorkel, one (1) knife, one (1) spear gun and one (1) BC-vest (cannot contain air cartridges). The charge is $75 on all flights. These items must be placed in a container that prevents damage to the equipment. Like all passenger airlines, we are prohibited from transporting scuba diving tanks, even if they are empty."
Looks like an old and flawed policy. Why were you transporting a spear gun? Then way I learned English, you would have to have all those things listed with the conjunctive "and" to meet their definition of Scuba-Diving Equipment. In any case, the answer is if it fits in a 50 lb bag and does not include any prohibited items, it is none of the clerks business. Although they say elsewhere in their policy that no refunds can be provided, it sure sounds like it would be worth a try, if for no other reason than to present your complaint.