irst, you probably know what it means, but just to be sure... You are underwater if first, you took out a mortgage on your home and you still owe money on it. And second, your home value -- what it would fetch on the market -- is less than the amount that you owe on the mortgage.
Being underwater means that if you were to sell the home, you would need to bring money to the closing to pay off the loan.
If you own your home outright with no mortgage, you may have lots of concerns about home values, but this is not one of them. And if you are not far underwater, it may not be much of a problem -- your home value might be rising fast enough to get you into the clear soon.