It appears that California (the state your question is posted from) protects people from deficiency judgments for purchase-money judgments on their residences; it's not clear that this would apply to you in the case of your "vacation condo," but it's worth looking into. You should consult with a local real estate attorney to see if you do, based on the specifics of your case, have any protection from a deficiency judgment. (Note that Hawaii, which is what your comment about the Big Island suggests, does not have any such protection and allows deficiency judgments, so if sued in a Hawaiin court, you'd not have the benefit of CA law.)If you do not have any protection, such as discussed above, then there is no other way to protect yourself--IF you walk away from or default on a mortgage, then if there is a remaining balance after the property is sold after foreclosure, the lender may go after you for it. As a practical matter, they often do not, especially if the amount is relatively small (a few thousand dollars underwater; maybe even $15k to $20k or so)--but they have the right to do so if they want. There is no way to prevent them from doing so, though you could then file for bankruptcy.