There are two main considerations in swimming:
Most improvements in swimming are made through improved technique, especially with less experienced swimmers. Therefore there should always be a strong focus on technique especially early on. In the beginning, strokes per length would be the best measuring
stick for improvement. Some swimmers may start out taking as many as 30 strokes per 25m then as they improve they should be able
to get it easily below 20. Ian Thorpe easily does 9!
The best drill I know for improving strokes per length is fist swimming. As it sounds just make your hand into a fist and swim. This forces you to use the rest of your arm most efficiently and when you start using your hands again you really feel you are getting a “good grip” on the water.
Why would we want to spend time training for the swim when it is the shortest leg of the race (especially in Ironman) when a good swim may only earn us a few minutes. The reason is cumulative fatigue. We may only come out of the water a few minutes faster, but we will be much fresher saving needed energy for the bike and run to come.
When I was at school I used to ride from the swimming pool to school and I could definitely feel the fatigue from swimming. I think CYCLING AFTER EVERY SWIM session also helped me ADAPT QUICKER at the start of the bike – another reason why I was one of the fastest triathlon cyclists.
HOW MUCH TIME you put into swim training depends on your goals. If your main events are Olympic or Sprint distance races swim training is essential even during the off-season.
If you are already a good swimmer and your main goal is Ironman or Half Ironman then swimming is less crucial during the off season. You may be better off focusing on the longer bike and run legs.
On the other hand if you are a very weak swimmer, it can be a very good investment of time to learn good technique and improve your confidence and strength in the pool.
Either way 3 times a week is usually enough to make good progress along with the other training you are doing. Always remember that swimming is part of triathlon. You need to work out how much time you need to put into it according to your current ability and goals.