It is not widely known but upper Bucks County, Pennsylvania is the world center for manufacturing small submarines. These aren’t the billion dollar babies built to launch nuclear weapons and destroy the world, but instead are used for recreation. (They don’t fire torpedos either. )
You can pick up one of these subs for under $5,000. They are great for deep lakes and can be used in the ocean, too. They only hold one person and can stay down for about two hours. The only problem is that you need a crane to put in the water, so a day underwater can be very pricey.