Correct, it actually says/suggests it can survive submerged in up to 1m of (toilet) water for 30 minutes. The 6s however, nope.
Not guaranteed, as mentioned, its a safety measure rather than for you lot to play with it - we can't forget that Apples Warranty did not change with the introduction of the iPhone 7, meaning if your device does die from water, then thats $1000+AUD down the drain! I do feel comfortable using my iPhone 7 Plus in heavy rain though for relatively brief moments, and this was NOT the case with the 6s.
Anyways as for you OP, you know what, I was once in a similar position, I'm a former 6s owner and I upgraded to the iPhone 7 Plus using the iPhone trade in program. I dont regret any bit of it. The new dual cameras can be enough of a reason to upgrade on its own, but beyond that I appreciate the sleek Jet Black colouring, (and yes I can see some of it, Apple Silicone Case leaves the bottom exposed

) the stereo speaker system, and Wide Colour as I'm an avid photographer. The real selling point of this phone though for me would be the battery life. Id say its comparable to the iPads, functionality wise - doesn't buckle under heavy pressure, unlike the 6s. Think about the hardware too - iPhone 6s is already almost 2 years old......how time flies.
If you have any other questions about 6s vs 7 Plus, feel free to PM me.