We were playing a fun game last week - below is a video of it. We use some clocks that beep and we can set them to intervals. We use these daily at practice but we can also program them at random numbers, which is what I did here.
The mission is to get to the first yellow lane line ring, which is about 8 yards from the wall, and get back. No other rules. Just get there, touch it, and get back. In the video you can see that we have two swimmers per lane (with others playing at the other end of the pool). I have also played this game with a stretch cord across the lanes where you could have 3 in a lane play - just touching the cord instead of the lane line ring. You can also use a whistle or "GO!" instead of the clock.
They have to listen and pay attention as well as swim fast. In this video they have already been doing this for about 10 minutes - so many of the swimmers are pretty tired! They love this game though. Sometimes they get 3-5 seconds rest. Sometimes 10-15. Sometimes 45 seconds rest! Sometimes we play for 5 minutes. Sometimes 10. Sometimes 15. They never know what is coming. With years of swimming up and down the pool endlessly we try to come up with fun ways for kids to enjoy coming to the pool. Sometimes we might make them streamline kick to the marker or something like that, but it is also fun to just fire up some swimming!
You can come up with different ways of motivating them (Gold/Silver/Bronze) for the first 3 swimmers on each round. One time we played after the sun went down and the 1st boy or 1st girl got to pick the color of this changeable LED lighting thing that we have around the dry erase board. If lanes are crowded you could have one lane with 10-15 swimmers do a continuous 50 swim/50 kick and another group spread out at both ends playing the game. After 5 minutes you can switch groups.