Here is a US Coast Guard site summarizing EPIRB recommendations which discusses and defines the types, ranges, uses, and what not to buy or use:
As mentioned by a previous poster here, note the number of items or classes of EPIRBS with: "No longer recommended."
Having sailed in both harbors and out on the open ocean, where there are no mountains sticking out of the water or other obstructions, usually, EPIRBS are useful and proper equipment is recommended by the Coast Guard.
Poor, low quality, defective equipment is worse than having nothing because it creates a false sense of security at precisely the moments when that is the last thing one needs -- like a choice whether to turn around and go back in rather than keep going into heavy weather or current, etc.
But, we were always warned that even on the open ocean with no obstructions to interfere with the radio signals the signals by themselves will not get a person out of trouble if they are caught in weather / seas that do not permit rescue or if a person is in the water without the right gear (and even with the right gear like Mustang body suits and flotation its iffy in cold water in the best of circumstances).
Thus, having the proper gear, sailing only in conditions for which we were trained or had experience, and making sure the boat, gear, and we were in proper order was the key. Avoiding, not inviting rescue situations.
As I recall we were given about 45 to 55 minutes maximum in the water w/o technical gear but with offshore pants and coat with flotation, if lucky, in the Pacific off the San Francisco coast before hypothermia would become a real problem. In that situation an EPIRB could be life saving because it would cut down the search time and part of a search and rescue operation.
The following is from the Mustang Survival web site, at:
"Water Immersion and Hypothermia
The human body is fairly well adapted to survive in very cold air for many hours; however, the thermal conductivity of water is 25 times greater than air. This means that the survival of an unprotected person in water below 10 degree Centigrade (50 degree Fahrenheit) beyond an hour is very unlikely. This is reduced to minutes as the water temperature approaches freezing, which is quite common in northern or southern extremes during winter.
Our body’s response to sudden cold-water immersion is universal. If unprotected, within 2 seconds of hitting the water, the body goes into “cold shock” and the physiological reaction includes the gasp reflex, hyperventilation, difficulty holding the breath, increase in blood pressure, and rapidly increasing heart rate. The extent of this cold shock depends on how much of the body surface area is exposed to the water.
Between 2 to 30 minutes after immersion, an un-insulated human body becomes incapacitated. A person may have difficulty swimming, lose functional abilities and manual dexterity, and experience muscle cramping. Unfortunately, swimming only speeds the onset of hypothermia.
After 30 minutes, hypothermia sets in and the body’s core temperature reduces to a point where a person loses consciousness and eventually the heart stops beating. For more information on the effects on hypothermia, please click here."
The Mustang web site also has a great video at that web page showing what happens and what goes into a rescue, the kind of gear that is designed to prevent hypothermia on the seas, and is fascinating IMO.