https://shadowspear.com/vb/threads/itc-prep-advice.21365/
The link above is a thread concerning preparation for ITC. The advice given therein should suffice for Marines attempting either MARSOC's selection or BRC, as both programs are somewhat similar from a physical perspective. I cannot say for sure what the current swim stroke requirement is at BRC. However, it
is safe to say that you will be required to swim longer distances, with or without cammies, utilizing proper survival strokes. Survival strokes are the side and breast stroke. Also, expect to do shorter "races" and "pays to be a winner" type PT swims utilizing the crawl stroke, or freestyle, as it is typically the fastest stroke.
Freestyle/crawl stroke: Pros- Fastest stroke. Builds cardiovascular capacity. Cons- Hardest to master the technique and puts a lot of wear on the shoulder joints.
Breast stroke: Pros- Moderately fast. Builds cardiovascular endurance. Cons- So much forward propulsion comes from the frog kick that groin injuries are not uncommon. Still wears on the shoulders.
Side stroke: Pros- Conserves energy. Can be maintained almost indefinitely. Easy to learn. Cons- Relatively slow.
If there are any recent BRC grads, or BRC instructors reading this, feel free to correct me or caveat as necessary.