In theory, if you applied the replacement adhesive 100% correctly, it will retain its water resistant properties. However, I would never deliberately expose any device to water, as you can never be completely sure that all the seals are perfectly intact.
When using the iOpener, you may need more than 30 seconds, especially in compact or low wattage microwaves. Take this slowly, though, and be extremely careful, as it is easy to overheat your iOpener. Remember, you can always heat it up a tad more, but you cannot undo overheating it.