When the COVID pandemic hit and public facilities shut down, many people were cut off from their gyms, schools, pools, etc. and faced with obstacles to improve or just maintain their fitness. As of this writing, it seems the worst of the pandemic may be over, but things are not yet back to normal. And there will always be challenges for some, situations where fitness equipment or facilities just aren't available, but you still have a program to follow and goals to meet. This is a discussion of ways to keep training effectively, even with limited resources.
Fortunately, the most important activity to prepare for Naval Special Warfare (NSW) selection is probably the easiest to do on your own, with no special equipment or facilities: running. Lace up a pair of shoes and step out the door. Depending on where you live or what transportation is available, it may be hard to get to a suitable running environment. Remember variety is beneficial for a running program, so ideally don't run the same few routes over and over. If you are confined to your immediate neighborhood and can't readily get to some of the different terrains that will best help develop your overall running ability (hills, trails, sand, etc.), be creative by improvising routes that include more than just the sidewalks and streets around your house. A route that includes a grassy stretch (maybe a nearby park or street median), a sandy patch (maybe at a playground or along the shoulder of a road), and a few inclines (even artificial ones like a ramp or set of stairs) would check a lot of boxes for a robust conditioning run.
Swimming is a tough activity to substitute. Unfortunately, you can't mimic swimming without water. If you don't have access to a pool, maybe you can get to a lake or the ocean. Be safety conscious and take precautions, such as swimming at authorized beaches with lifeguards, or at least swimming with a buddy. An effective technique given the absence of physical water is visualization (mental rehearsal, one of the Big 4 mental strategies used in NSW; see my blog post "Mental Strategies You Can Use"). In a quiet environment where you can meditate without distraction, review in your mind the various technical aspects of the Combat Sidestroke. Study some technique videos and think about what you see. Concentrate on aspects that have given you trouble before. Use internal and external perspectives and use as many senses as you can (it's not just "visualization"; for instance, think about how the water feels or what a good stroke sounds like). Remember that it takes practice to get good at mental practice! Mental practice won't totally replace physical swimming, but it can certainly help keep your technique from eroding too quickly until you can get back in the water.
Adding cross training to your routine is a good fitness strategy in general and if you can't use the pool and you can't use the gym (no rowing machine, stair climber, elliptical machine, etc.) then tune up your road bike to get some mileage and build your cardio fitness. Maybe you have a pair of in-line skates, or a jump rope. If you can't even leave the house, you could find a sturdy box about ten inches high and step up and down on that for 20-30 minutes (alternate legs so you're not stepping with the same leg the whole time). Cross training can't replace running or swimming, but it can complement them, and there are plenty of good cardio activities that are much better than nothing at all.
Calisthenics can be done anywhere, so you can always work on your push-sit-pull routine and plenty of other exercises to round out your program as well. You may need to be creative to do some exercises, such as using a clothesline support for pull-ups or a chair for dips. For more resistance than body weight and a more intense strength workout, there are plenty of options even if you don't have dumbbells, bars and plates, or other standard weightlifting equipment. Look in the basement, garage, or even the kitchen for some "equipment". Bags of sand, dirt, seeds, flour, etc. can be used for exercises like squats, overhead press, or row pulls; canned goods of various sizes or jugs of laundry detergent can be used for shoulder work (rotator cuff, traps). Other possible makeshift weightlifting equipment could include tools such as hammers (especially sledgehammers), containers such as water cans, or items such as bricks or cinder blocks. Don't be foolish and lift awkward objects that are hard to grip over your head until your arms give out but be clever and creative when looking for things to lift. Elastic bands are inexpensive and versatile. A truth about strength training is, your muscles don't know or care what you lift, if you challenge them with some resistance, they'll respond by getting stronger. Try not to compromise the general guidelines for technique, such as using controlled movement and working through a full range of motion but make alterations as necessary to perform the desired exercise. An 80% solution is better than 0%.
In the absence of an ideal situation, use available resources and some ingenuity to create a "pretty good" training environment until "great" becomes an option again.
Naval Special Warfare Assessment Team