Underground storage tanks can easily collect water. Water can enter the tank by condensation; as temperatures rise and fall, moisture-laden air moves in and out of the tank. Moisture can enter via the tank's filler pipe, especially if the cap is near, at, or below ground level. The water that collects in a fuel tank will eventually corrode the tank, causing it to leak. Underground tanks are especially difficult and expensive to move, but it is equally undesirable to have a leaky indoor tank. Check your tank often for water and remove it if necessary.
Soak up the water in the bottom of your tank with a water-absorbing alcohol-based product. Ask a professional for a product recommendation. If water is a persistent problem, and you can't seem to solve it, you may have to add one of these products every time you have your tank filled.
Insert a water-absorbent "sock" into the tank. These socks are relatively inexpensive and will sink to the bottom of the tank and soak up the water that has settled. The sock is attached to a long string that stays outside the tank, and you retrieve the sock when it is full. If it has not absorbed all it can, you can easily reinsert it.
Pump out the water. For large amounts of water, there is no real solution other than having it pumped out. This is a service provided by your oil company, and one it should be competent to perform.
If you have an underground tank, test it regularly for water accumulation. That water will rust holes in your tank, and you do not want an underground tank to leak. The cleanup can be very expensive, and replacing a tank is not much cheaper. Be vigilant about keeping the oil in the tank and out of the ground.