I'm not sure thinking of the robots as "increasing safety" is a useful mindset. They increase the chance of rescue. If you needed one in the first place, it's because you unintentionally neglected safety well before they deployed the robot to you. Anyways.. as an aside.. if it's just about getting flotation devices out to them faster, why not a "life jacket cannon" mounted to the beach? If you really want to make it…
They also won’t have the energy to put the life jacket while already in the water. And while life jackets properly equipped around your torso make you float correctly, barely resting your arms on a life jacket is not terribly buoyant.
You need to account for wind, not just to aim, but also because the life jacket will drift away. And as it happens strong winds are responsible for rip currents and waves which is very often what causes drownings in the first place, so wind is the default environment, not an edge case.
Waves and surface currents have a strong chance of carrying away the jacket before the drowning person can get to it.
A system that can be course corrected to account for all of those parameters seem the best, its rigid, so will provide buoyancy even when just holding on the device, it’s also big enough to support more than one person, many drowning events involve more than one person (often someone that was initially trying to help end up getting themselves in trouble).