Need to change battery? Here are some teardown guides: ifixit.com
For more easily replaceable batteries? You can get a Fairphone (www.fairphone.com) if water resistance is not important to you. Fairphone 4 is IP54 so it is somewhat water resistant, but not submerge-resistant. If you want repairability, you can get a fairphone.
There are also phones with both water-submerge resistance and replacible batteries (but not as repairable as a Fairphone): Samsung Galaxy xCover (www.samsung.com/us/business/…/galaxy-xcover-pro/)