The 6s isn't waterproof.If its one of the waterproof ones it will never be the same because the way the glue is put on and has to be peeled out to change the battery. Im sure Apple has some kind of log of when they replace batteries on phones so if something goes wrong they will take care of it. I would press for a refurb if possible.
Rechargeable batteries at some point will stop recharging why do manufacturers stop you replacing them simply ? Apple are a prime culprit, annoying as hell, just allow people to replace them, simples, not unsolder mainboards or have to deconstruct an entire pos device just to replace a godamn battery !!!
If you warranty the battery for life then fine otherwise fo and let us sort it from China