Great thinking & great use. Wonderful. So good you have to ask how come this was not being done already, and why one's self did not think of it.

I believe a lifeguard on a jet ski would be possibly faster, much more reliable and probably more cost effective and does not even take into account weather not to mention if the victim is unable to assist once help arrives.

2.8 miles is not very far considering. I would think adding a tether spool between the drone and the drone pad would add retrieval ability. It should only take one or two hundred pound break strength to pull the victims to shore so should not add a huge weight to the drone especially using the newer advanced fibers like Zylon or Spectra.
So to summarize: Follow the makers idea and make the drone float, attach the drone to shore with a shore mounted reel, attach the life preservers to the drone with generous line and then reel the whole thing in once the victims are aboard the preservers.

Rehab I can see from the video that it is faster then a jet ski when you consider launching the jetski and new jet ski's start at about $12,000. This would be easier to store and set up. A windy day could be a problem though.

new toy for the NSRI?

Life saving robot sounds great. But this a DRONE, that is controlled by a life-guard. So you need an extra person to control the drone. And when do many problems occur? When the weather turns bad. Then this light-weight flying drone can be very dangerous, and might even injure people. Still nice idea.

Who will rescue the rescuer? My feeling is that the Pars will no stay airborn in strong winds It will be flattened to the surface.

Not so many swimmers go out in rough weather.
A thin but strong line could be attached to the drone, then the swimmer in distress could be hauled back to safety.

What good does it do to drop life preservers to victims of drowning? If you have drowned, you have already died.

Didn't see it in the article, but it doesn't appear as though these life preservers were each equipped with a GPS device and a blinking water-proof light. Those would be crucial in an emergency situation where current and weather were a factor.