Water pressure goes up pretty quickly with depth which means items that are completely water resistant at very shallow depths can quickly lose their water resis

Water pressure goes up pretty quickly with depth which means items that are completely water resistant at very shallow depths can quickly lose their water resistance. Plus, any seals may not be statistically perfect and could degrade over time. The reason why Apple hasn't marketed the claim is likely the one most people assume: They don't want to be responsible for warranty repairs for any water damage until they're confident about their process.