I have an old Series 5 that I use a backup watch to my primary. The battery dies fairly quickly. It's probably not worth much to sell, and Apple charges $299 to replace the battery, which is more than it's worth. There are battery replacement kits on Amazon for like $30. I've repaired many Apple products before, so I'm not too concerned about the work required. I'm just wondering if it will ruin the water resistance by doing this? I use this watch a lot at the beach/pool, so it will be submerged/splashed a lot but not very deep. Thanks