The classic way to convey the error that exists in a measurement is a formula that shows the obtained measurement (X) as a sum of a true score plus some error. One version of this formula is: X = T + e. With this formula in mind, obtaining more precise measures depends on reducing error.
  Mega is the prefix meaning one million, therefore there are one million (1,000,000) gallons in a mega gallon.
1 cubic kilometer of water contains 1 trillion liters of water. Therefore there are approximately 1,300,000,000,000,000,000,000 liters of water in the oceans, which is 1.3 sextillion liters. To get to gallons, there are 3.78541178 liters in 1 gallon. Doing the math, that means there are 343,423,668,428,484,681,262 gallons of water in the oceans.