From Openwaterpedia

verb - To descend means that each swim repetition in an interval set performed in a pool or an open body of water is faster than the previous repetition. For example, in a set of 6 x 50 descend 1-6, the swimmer will swim six 50's with each 50 gradually getting faster. If the set is 6 x 500 between piers descend 1-3, 4-6, the swimmer will swim gradually faster on the first three swims between piers and then slow up on the fourth pier swim and then gradually get faster until the sixth pier swim. Th interval generally stays the same, giving the athlete more rest as their swimming time descends (gets faster).

The swimmers had to descend on a tough set of 6 x 200 on a fast interval.