Luckily I have been exposed to many different swim groups in my adult life. Every coach, both men and women, had different philosophies on how to design their sets based on warm up, main set, and cool down. Experience from going to all these practices has given me an appreciation of how to build a swim workout. I’ve listed what the purpose of the common sets you see below.
Sprints: These SHOULD be placed at the beginning of each set after your warm up. The goal is to be fresh so maximum speed can be generated. If they are placed at the end, they are more like intervals.
Intervals: The idea behind an interval is to go faster than you normally would in a race but have sufficient time to recover (unlike a race). The recovery time will depend on the time of year but the athlete should be able to hold a fast pace. The amount of intervals depends on the year and the experience of the athlete. A problem I see often is when a swimmer(s) cannot sustain the pace on the amount of recovery given so they get in the way or are forced to stop or just keep going. This is counter productive as the athlete is not getting sufficient rest nor going fast enough so it becomes an edurance swim (see below). Generally it is up to the swimmers to seed themselves in the correct lane but sometimes the coach should take control.
Endurance swims: There are not many coaches who prescribe long working sets (500m+). The obvious reason is space as athletes may all run into each other. The second reason is boredom as many swimmers look for short fast intervals or sprints for conditioning. I think endurance sets are very helpful for improving stroke mechanics and aerobic conditioning.
Tempo swims: Rarely do I ever hear a coach talk about tempo. The premise is to go at a speed that is slighltly slower than race pace but have shorter recoveries. This allows the swimmer to ‘think’ about their stroke instead of just full out as an interval should be.
Warm up: One specific coach always had 1000m regardless of day it was. Another had 200m and another 400m. The idea of a warm-up is to allow your muscles to heat up and expand, so you are able to do work. I do not think there is a designated number and I think variation is good. My only suggestion is sometimes a short warm-up (<400m) for some athletes will not allow their bodies to be ready for harder work.
Cool down: Depending on the workout, a cool down can act as two things: the first is the common cool down so your muscles can return to a lower state of excitation and the other is to improve aerobic conditioning. The latter idea would be used if for example you did, short sprints (so very hard), intervals (again hard), and then have a long endurance based (500m+) cool down at the end. The swimmer after a hard main set will be tired but adding in a long endurance set at the end will help improve their aerobic conditioning to simulate a long race (these sets are recommend for triathletes).
There are just a few set recommendations for swimming. The same principals can be applied to most other sports as heavy or fast training should go at the beginning and then slower or longer sets go at the end.