The remedy - a slow start to the lesson, based on the slow starts I have have seen and read about in many Elementary classrooms. At the start of the lesson, as soon as the bell goes I open the pool gate and the students know they have 10 minutes of changing and free swim time before the lesson activities start.
Students use this 10 minutes in a variety of ways. Some dip their toes into the water, as they gradually enter the pool; others dive straight in and swim a few laps; some practice skills that we have covered; while most play with their friends - Marco Polo, tag, relays, jumping and diving in and this week, one student brought in waterproof uno cards to play!
From the student point of view this change in structure means that they have a guaranteed free choice time every week. This has resulted in many students arriving already changed ready to jump straight into the pool and everyone getting to the pool much earlier. And from my point of view, we have actually increased the instruction time of the lessons - so everyone wins!