What do you mean by underwater mortgage? Are you behind in your payments for this mortgage?
If you are behind in your payments most likely the mortgage company (bank) has reported this to your credit report. You won’t get another mortgage if you are a slow payer or if you are behind in your current payments for your current mortgage.
If you mean you owe more than what you can sell your current home, and want to buy another home, that’s different. Your bank can help you figure out how to pay off your home and get another mortgage for the second home by assuming the debt.