My iPhone 7 plus was on 1" of water for 30 seconds and was dead within 5 hours. When I returned it to apple (who sold me a new one at a discount) they stated they have seen a number get returned and it's not waterproof and my experience was the norm from what they have seen/heard. Bases on on how my phone died after such shallow and short exposure I would consider this a HUGE exception and likely to never be repeated. The phone is not waterproof not even close.
Perhaps we could focus on somethig more practical than storing electronic device under water. How about making the glass not breaking on some modest impacts?