Picture a standard torus shaped life preserver on a rope used to prevent drowning.
Now picture the same torus incorporating a hollow tube inside of it, from middle of the tube to almost touching the outermost periphery of it. Then visualize a shuttle within that hollow tube which is very heavy.
shuttle itself will be much smaller than the diameter of the tube. It will be bottom-heavy and will contain an upper weighted rod which can be swung right or left without over-balancing the shuttle itself within this tube.
Outside the torus attached to its center-line are... curved triangulated cup shapes on swing-arms so that, no matter which way the torus spins, water pressure alone will cause them to swing one way or the other and grip the torus so as to displace water in only one direction.
Engage the shuttle from a remote control and the torus will stand up within a revolution or two, the swing-arm cups snag water and will paddle-wheel that life-preserver up to whatever speed its little rechargeable motor can attain.
Displacing the inner-shuttles' counter weight left or right will cause the shuttle to track inwards or outwards within the tube and create a counter-tilt to the whole contraption allowing it to be steered from the dock or a boat, or from shore to the aid of someone drowning.
Now... to get them back...
The upright mobile torus has a saddle on casters which rides freely along its inner curve.
The stirrups of the saddle are elongated so that, on a real horse they would drag on the ground but behind a rolling doughnut on water, they just drag an aqua-dynamic, handle-covered life preserver on spring-loaded retractable straps.
You toss the life preserver into the water.
Disconnecting it from its power source/charging station activates remote control function and it tears-ass across the water to those in need of help.
If the person can help themselves in advance of human intervention then fine. The wheel reverses and distances itself from the drowner before dousing them in a rooster-tail of water they probably don't need right then as it tows them to safety...
...or it waits patiently for humans to arrive and load up those who can be fastened and taken back to safety faster than swimmers alone would allow.