Let me get this straight: you don’t want tropical, but you can’t wait six more months until summer for your next domestic dive-cation? You're dedicated, Adam. I’m assuming you’re not interested in ice diving in the Great Lakes, because no one in their right mind goes to Green Bay or Detroit in the winter if they can help it. (Then again, no one in their right mind cuts a tiny hole through two feet of ice and then dives into it for the fun of it, either.) In which case, here are your two top non-tropical but non-frigid winter options in the US—one freshwater and one saltwater.
Freshwater: Bonne Terre Mine, Missouri
No one goes to east-central Missouri in the winter for fun, either—unless they're among the 15,000 people who make the pilgrimage each year to dive the Bonne Terre Mine. One of the single largest sources of lead in the world for more than a century, the old mine was closed in 1962 and filled with water. It’s now the largest freshwater dive resort on the planet. The water stays a constant 58 degrees Fahrenheit, and the shafts are illuminated by 50,000 watts of artificial light. There are 24 dive routes, winding beneath broad arches and down staircases at depths of 40 to 60 feet, with visibility stretching to 100 feet. You won’t see any marine life, but diving there feels like taking an underwater tour of a forgotten world. Cost is $70 per dive, and rooms are available on-site.
Saltwater: Flower Garden Banks Marine Sanctuary, Texas
These three towering coral pinnacles rise to just 50 feet below the surface, about 100 miles off the Texas and Louisiana Coast in the Gulf of Mexico. If you’re willing to brave the seven-hour boat ride out to the area's 27 dive sites, you’ll find the reefs overflowing with hundreds of species of fish and reptiles, including hammerhead sharks, manta rays and loggerhead turtles. The winter season runs from February through April, and temperatures bottom out in the high 60s, with visibility of 50 feet or better. Trips usually last at least two days. Texas Blue Water Safaris charters tours from Surfside, Texas, and Fling Charters from Freeport, Texas. Prices vary based on group size, but expect to pay around $600 for a trip.