Even though often these devices are labeled “waterproof," they're really water-resistant — very few electronics are truly waterproof, at least in any official sense.
The International Electrotechnical Commission (IEC) developed the International Protection (or “IP”) rating in 1976 to give you an idea of a device’s water resistance. One determines dust resistance (out of 6) and the other determines water resistance (out of 9).
Apple’s iPhone 11 Pro and iPhone Pro Max are both rated IP68, with Apple boasting water resistance for up to 30 minutes at a maximum depth of 4 meters. That makes them some of the most water-resistant phones on the market.
All Apple Watch models are splash- and water-resistant, though the Series 2 and newer are all submergible up to 50 meters, an upgrade from the earlier models. Apple claims you can use them swimming but recommends not using when scuba diving, water skiing, and other high-pressure activities.