The battery size and device's functionality are the two most irrelevant factors. Water doesn't care if the thing has Bluetooth.
Whether it's a watch or a phone, you don't want water or moisture inside. Simple as that. Watches do have the advantage that they can be built so that they're 100% filled with a lubricating fluid, making them more or less immune to outer pressure.
Whether it's a watch or a phone, you don't want water or moisture inside. Simple as that. Watches do have the advantage that they can be built so that they're 100% filled with a lubricating fluid, making them more or less immune to outer pressure.