In general, I am 'under water' is similar to I am 'drowning'. In general, it means that you are overwelmed by a situation. Specifically in the US since the recession it jas come to mean that the market value of your home is less than the balance on the mortgage loan you owe. This is a special usage.

'I'm under water' is not an accepted idiom, so you would have to look at the context to know what is meant.