Well provided that all of the ports are correctly covered by the port covers, the Sony Xperia Z Ultra will continue to function underwater up to a depth of 1.5 meters for up to 30 minutes. Of course you need to take care that the water your phone comes into contact with isn't salt water or contains other chemicals such as chlorine found in pools, but practically it means that if it happens to come in contact with a liquid, your phone should be relatively safe and can be washed off under a tap. Liquids other than freshwater may contain chemicals or dirt which may play havoc on the rubber seals or speakers and can cause potential damage especially if left on the phone for an extended period of time.
Read and see more official test result photos after the break...
Official test results for the IP55 and IP58 tests of the Sony Xperia Z Ultra
And courtesy of Sony, here's a few photos of the actual test process followed: