The Great Lakes (Huron, Ontario, Michigan, Erie, and Superior) comprise the largest unfrozen freshwater system on Earth, and have a surface area greater than the United Kingdom. The surface area and average depth of each of the lakes is as follows:
H - 59,596 km^2 59 m
O - 19,477 19
M - 58,016 84
E - 25,719 19
S - 82,413 397
The volume of water (in cubic kilometers, or km^3) for each lake is the surface area times the average depth:
H - 59,596 km^2 (.059 m) = 3516 km^3
O - 19,477 (.019) = 370
M - 58,016 (.084) = 4873
E - 25,719 (.019) = 489
S - 82,413 (.397) = 32718
The total cubic volume of the Great Lakes (in cubic kilometers, of course) is the sum of the last column, or 41966 km^3.
Now 1 cubic kilometer is equal to 33,814,022,701,000 US fluid ounces. That means 41966 cubic kilometers of water is equal to 141,903,927,670,000,000 fl oz., or 1.419 x 10^17 fl oz, or about 142 quadrillion fluid ounces. Splish splash!!
While I suspect this is a question from your textbook that could be found by reading the assigned chapter, I have included some valuable links below that you can access to find a list of all the lakes in the United States, and a calculation of how much water is in all the world's lakes.
According to the US government, there are 176,400 cubic Kilometers of water in all the world's lakes. After finding out how many of those lakes lie in the United States, you would then have to convert cubic Kilometers into fluid ounces.
For your use in the calculation, know that 1 cubic kilometer equals 3.38140227 x 10 to the 13 Fluid ounces.