We've all been told to avoid standing under trees during lightning storm over risk of getting struck by lightning. The same applies to standing in water. Whenever a storm approaches, outdoor swimming pools are cleared for the safety of swimmers.
Don't get me wrong, it is very important to avoid water during a lightning storm, but unfortunately this leads to the misconception that water is an excellent conductor of electricity. The misconception states that lightning striking water will easy conduct through the water to anyone standing in the water and thus electrocuting them.
Although you are very likely to be electrocuted if lightning strikes water you are standing in, it's not the water itself that increases conductivity. Water is a very poor conductor of electricity. The catch however, is the difference between pure and impure water. Pure water is a very poor conductor of electricity but we rarely come across pure water. Most water is filled with minerals (e.g salt). The ionic compounds from these minerals are what conduct electricity and make it dangerous to mix water and electricity. Even tiny amounts of these compounds greatly increases the conductivity. Thus fresh water which we normally think of as unsalted, will easily conduct electricity through ions.