I think 90% chance of apple going for the EU-Specific model just like they did for Digital markets act, 10 chance of screws, 0% chance of actually popping the back cover off with bare hands.
For Samsung, 50% chance EU-Specific models 40% chance screws, 10% chance back cover tool-less removal. (Edit: typo)
As for other smaller manufacturers, probably just 50%/50% either screws or tool-less back cover removal.
You seal everything but the battery in one watertight compartment. The battery is a sealed, self-contained package, in a second compartment. Dunk the phone, the only thing that gets wet are the battery contacts, which are protected the same way that earbuds with magnetic chargers are protected: if the contacts get wet, the battery shuts itself off until it has been dried off.