Is iPhone 6S Plus water resistant?

No. The iPhone 6s plus has no degree of water resistance whatsoever. Water resistance is a feature of iPhone 7 and later.

However, even iPhones that feature some form of water resistance are not meant to be used under water. Apple is very clear about not submerging any iPhone.

See here: About splash, water, and dust resistance of iPhone 7 and later - Apple Support

Even more reason to not get it wet.

It is water resistant, but it is NOT water proof, thus is you submerge the phone in water, be prepared for it to die and require replacement.

Actually, the 6s has no rating for water resistance whatsoever.

That started with the iPhone 7.

Exactly! :P

You're welcome.