Level 6 Valued Member
I guess that these are 3 separate events, that is* Swimming (100m / 50m underwater / hand and feet bound)
1 - swim 100m with a time limit.
2 - Swim 50m underwater in one breath.
3 - do something in the water for certain time with hand and feet bound.
How are you at the swim? That is one test that needs to be specifically trained. Strength/endurance have little to do with it, it is a technique issue. Since your feet/hands are bound, it may be a good idea to have a friend with you when you jump in the pool! It's a mental test as well since it will be quite stressful being bound in a pool.
I was never special forces or anything coming close to that, but I played waterpolo at the provincial level for 12 years and did 2 years of underwater hockey, in addition to swimming a lot of the years I was not participating in these two sports, so I know water. As @Grayland said, these are all technical and require practice, which may be difficult with COVID, depending on where you live and the availability of pools.
I would say one thing though, don't underestimate 50m underwater without breathing, if that's the test. This in itself is not an easy thing to do. If you have never done it, you need to practice it. It requires a fine balance of movement efficiency, mental fortitude and training to let the body know that you will not die if you don't breathe for 10 second. I remember one year we were training where there was a 50m pool and the coach one day asked us as part of the warmup to swim underwater as far as we could. I stopped at about 42-43 meters as I realized there was nobody with me in the water so far from the starting wall, but none of the other players, even many who were much faster swimmers than me, made it that far. I think I would have made it to 50m that day, but there is no way I could do it now without training for it. That year, I had trained over the winter with a coach who made us swim laps with pyramidal number of strokes per breath (started at breathing every 3 arm strokes, then 5, then 7, then 5 then 3. I think some even did 9, but that was too much for me), and my guess is that this helped a lot, but I cannot be sure. Also, doing the test in a 25m pools with the ability to push off the wall at each end, or doing it in a 50m pool with a single push is very different. If you practice in a 25m, pool, make sure that you can go more than 2 laps.
Practicing this is also dangerous. If you come anywhere close to your limits, there is a chance that you can drown, especially if you hyperventilate a lot before going. Hyperventilating inhibits your urge to breathe (which is mainly caused by carbonate buildup in blood due to CO2, and not lack of oxygen as many people believe), and you can simply faint without realizing that you were low on oxygen. Never push this alone in a pool. Always have a trusted person with you who knows what to do with an inconscious person in water.
@Pet likes to dive (and a living Google with the ability to post relevant links to almost any fitness related question on demand), so he may have good pointers for you.