Water Damaged / Expired US Passport

The State Department has this to say, in its FAQ:

My passport has been damaged. Can I continue to use this passport?

If your passport has been significantly damaged, especially the book cover or the page displaying your personal data and photo, you will need to apply for a new passport. Conditions that may constitute damage requiring you to replace your passport include water damage, a significant tear, unofficial markings on the data page, missing visa pages (torn out), a hole punch, or other injuries.

Normal wear of a U.S. passport is expected and likely does not constitute "damage." For instance normal wear includes the bend of a passport after being carried in your back pocket or fanning of the visa pages after extensive opening and closing.

If you need to replace your damaged passport, you will need to submit the following in person (See Where to Apply):

• The damaged passport;
• A signed statement explaining the damaged or mutilated condition of your passport book and/or card;
• Form DS-11; and
• All documents required by Form DS-11, including citizenship documentation (i.e., birth certificate).

Any water damage on the personal data page appears to qualify as damage.

Note that this requires you to appear in person at a post office or other passport acceptance facility, while regular renewals can be done by mail. Aside from that, there is not much difference.

After calling the Passport Agency's customer service line, they said the general rule of thumb is that if there's even a question about damage, then it is better to be safe than sorry and apply for a new one instead of renewing. Even if all of the information is readable.