Find someone who has one of those water rescue doughnuts, and ask if you can borrow it for this activity. In its place you can also use just about any other type of flotation device: A life jacket, a paddle board, etc. It should be something that floats, however, since the goal of this activity is to teach kids to toss something useful if someone is struggling.
Attach a thin nylon rope to the doughnut or other flotation device. (The thicker the rope is, the harder this activity will be for children.) Go outside and find some sort of natural divider: the edge of the sidewalk, the grass, wood chips, etc. Have the tosser and one helper stand at the edge of the divider. This will serve as the pretend boundary for the boat, shore, etc.
Next have 5 or 6 kids stand on the other side of the barrier, some reasonably close by, others farther away. One child holds the end of the rope while his helper tries to toss the doughnut towards one of the “swimmers” that needs rescuing. If he succeeds in getting it close enough that they can grab onto it (they can move their arms but not their feet) then they reel that swimmer to safety.
Give each child 2-4 tosses. If they snag one, encourage them to aim at one of the kids that is farther away nest time. Then switch roles and give each child a turn being the tosser, swimmer, and helper. When you’re playing this game, promote water safety by encouraging the kids to come up with fanciful stories about how they got into trouble in the water, and what can be done to avoid this.