Just consolidating comments here:
Wear insulated gloves.
Wear rubber boots.
Use an insulated tool to unplug the electrical wire.
Use a wooden stick or an insulated tool to flip the breaker into the off position.
Others are correct Standing Water can become electrified with out the circuit breaker tripping; so please do not listen to those who seem to claim the breaker will trip if the hot line comes into contact with the water.
Some commented that electricity finds the path of least resistance.. that is true, but when in water you are the least path of resistance.
Edit: I added an intro sentence to explain the reason for the information that I hilited.
To explain the process of how you become the least path of resistance (between the ===)
Contrary to what people think: WATER is NOT a Conductor.
Yes you read that correctly Water does not conduct electricity
The impurities in the water conducts the electricity - and you the person are an impurity of the water and become the conductor!!
Now for those that do not believe me - you get your distilled water in a clean bottle and do the continuity test; then add NaCl (Table Salt) and repeat the test. Or you can just search the web!
EDIT added a heading to this section of links
Real world examples of how people were electrocuted and the breaker did not trip the links below are provided:
Electric Shock at Pool
Electric Shock Drowning at Dock Lake:
Just to make sure I am being clear here: Yes the water in the basement can conduct because it has impurities in the water, rain water, city water, well water it all has impurities.. distilled water does not have impurities it does not conduct.
That said - the point of my explanations of water not conducting was to illustrate How that process of path to least resistance works.
How can the water be electrified and the circuit breaker has not tripped: No current path to return to source.
GFCI senses an Imbalance between hot and neutral (meaning an alternate current path) and so it trips - which is why you have them by code wherever you have the potential for water to be near Kitchens, Bathrooms, Pools, Spas , Hot tubs, Outdoor areas.
Now you as a person are conductive and when you are in that water (which is by nature impure); you are the path of least resistance to ground - much better than the impurities in the water ... you are more like a live wire from the water to the earth.
I tried to make these clarifications and points to help the op and also other commenters - like one commenter stated..
"how would just treading through the water kill someone?"
Well this is how it would kill someone - it would be electrified and find the person (impurity of water) to be the best conductor to ground.
And further to clarify the point that the circuit breaker does not always trip just because the water becomes electrified.
In other words I want the op to know regardless of those comments that there is a real danger that the op could be electrocuted if they adhered to those particular comments. Do not disregard the danger..
I do recommend the op change out his breakers or outlets for GFCI's in that basement.