Dead or drowned bees can still sting. sounds like you friends little bro touched the wrong end of the dead bee They were probably only looking for water and discovered the pool and sent a party to collect water. They drowned because they had nothing but the surface of the water to land on. doesn't sound like anything too unusual. Also wasps and hornets may have discovered it and drowned when getting a drink.
Most of the time wasps give bees the bad rep they tend to have. The average person tends to consider any flying insect that stings a bee.
There are some bad bees though, that can be either really aggressive or really defensive. Both are bad if you are trying to work them and defensive is usually ok for a wild hive unless in a location where it would be disturbed. In both cases though if you see one foraging they are not likely to sting you.
Sorry, I tend to get carried away when I start talking about bees.