Slightly longer answer: probably not
Even longer answer: While the evidence is largely anecdotal, Apple has certainly modified the iPhone design. These modifications seem to keep water out - to an extent, as found in various internet searches. Our iPhone 6SP isn't scheduled for delivery until tomorrow, so while we haven't actually seen these modifications personally, the internet has no shortage of 6S and 6SP teardowns and tests. Below is just one screenshot of a video on YouTube questioning this very subject.
According to IP standards (International Protection ratings), there are 9 levels of protection for the waterproof category and 7 levels for dust proof. Charts for these are show below.
Yet, there are many cases of water damaged S5's. A quick search of Google confirms it and we've seen them ourselves in the shop. So what's Apple doing?
Waaaay back on March 6th of this year, Forbes posted an article titled, "iPhone 6S Will Have A Surprising New Feature"
Just as Samsung ditches waterproof flagship smartphones, does the exact opposite – according to a newly filed patent.
The patent application – first reported by Appleinsider – suggests that Apple wants to experiment with waterproofing a future device.
So what's going on with the 6S, and why the secrecy, or lack of official comment by Apple? Perhaps Apple is doing what Samsung didn't and this is the largest scale consumer product testing under a wait and see approach ever undertaken. Apple knows people buy their phones just to put them through rigorous tests - including submersion in water, so why not make some changes to improve susceptibility to water damage, then release it without telling anyone and see how the market responds? Without an "official" water/dust proof rating or official statement about being waterproof from Apple, the phone manufacturer can sit back and watch what happens with their first attempt at a waterproof phone, then make changes for an official version later. Pure speculation of course, but until Apple makes a statement, that's all anyone has.