In the UK, many of these signs are in old disused and flooded quarries and mines or reservoirs. The issue with any of these sites is they are typically incredibly deep (like 100m~) and become deep very fast. I know of at least one reservoir where within 2m of the shore it drops to the point where you can no longer see the bottom (I'm not even sure how deep it is).
When it's hot it's easy to jump straight into the nice cool water. The problem is that the water being very deep takes a lot of energy to heat up, so even near the shore it can be several 10stens of degrees Celsius colder than the air temperature. This can lead to cramp, stitches or even cold water shock (see quote below from wikipedia).
In humans, cold shock response is perhaps the most common cause of death from immersion in very cold water,1 such as by falling through thin ice. The immediate shock of the cold causes involuntary inhalation, which if underwater can result in drowning. The cold water can also cause heart attack due to vasoconstriction;[2] the heart has to work harder to pump the same volume of blood throughout the body. For people with heart disease, this additional workload can cause the heart to go into arrest. Inhalation of water (and thus drowning) may result from hyperventilation. Some people are much better able to survive swimming in very cold water due to body or mental conditioning.1
In any of these responses it can become impossible to swim. If the water gets very deep very quickly, you don't have to be far from the shore to get into serious issues.
Most lakes, etc. in the UK don't have life guards or typically anyone near with a boat, etc. so it's difficult to help people in these situations. This, historically, has led to groups of people (mostly teenagers) drowning as when one gets into trouble, people rush to help and end up drowning too.
It's become a particular issue in the UK because we have lots of old (Victorian) industrial sites, many of which are flooded. For example, living in North Wales where I live is literally covered in quarries, and mines like this:
A couple of headlines from this article on the BBC:
381 people drowned in accidents across the UK in 2013
Gullet Quarry, in Worcestershire, claimed the lives of two swimmers during July 2013
The body of Conor McColl, 16, from Luton, was recovered from a former quarry in Clophill, Bedfordshire
Headlines like the above have led to a lot of legitimate and silly (that add no benefit), signs being put up by councils and local authorities. They simply don't want to get asked "Why didn't you do anything about this hazard" when/if someone dies.
