A pool toy going swimming for himself would obviously fill up with water before getting in. That way they would be neutrally buoyant, and able to swim. The rubber ring is simply there for buoyancy trim adjust.
Or two nozzles. Water goes in one, air comes out the other.
You could do it with one using a custom pump - remove some air, add some water, repeat. Or you could use the much simpler solution of asking a friend to help.
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