Replacing a battery on a waterproof phone.
Is it possible for me to replace the battery on this device while maintaining its watertight seal after the deed is done?
No. Factory waterproofing is a nanopolymer internal coating that's applied via pressure after the phone has been assembled. When you disassemble a waterproofed phone, you break that coating and weaken the protection. You won't fully destroy the protection; you're just weakening it at the places where you've taken things (including screws) apart.
On the bright side, some stores (like mine!) have the necessary equipment to restore that nanopolymer coating after a hardware repair has been made. It's inexpensive and quick to apply, typically taking less than 10 minutes.
I've personally have open the Note 8 and know that you can buy a replacement seal and Back glass and it'll cost you less than $15. It's only 10 screws to get to the battery after you use a heat pad to remove the back glass it's easy if you have a heat pad or heat gun or hair dryer you just need a lot of guitar picks or credit cards to slide underneath the edges all around it. P.S. the fingerprint scanner connector is on the right side of it and the camera lens cover assembly and that comes off with the rear glass as one piece.
Unfortunately no. To replace the battery you must remove the back which will break the water resistance seal. You should also be careful because those back are very fragile and prone to breaking when coming up and if they don't break the paint likes to peal.