I have never read any survival theme manga with this level of misinformation before. I couldn't go more than 3 pages without face palm myself.
For one thing, drinking raw water is a BAD idea. The most that bottle he brought can do is filtering the dirt but there's still germs and nasty bacterias that can't be filtered out. Fun fact, there's no such thing as a magic bottle that can disinfect all those germs for you.
And how did he got a conclusion that he's in another world? There's no indication whatsoever. If that's the first thing came to his mind after being stranded on an uninhabitated island, his sanity needs to be questioned.
"If this is an uninhabitated island, there's a chance there are people living over there"
Never mind what I said before. This guy nuts. His deduction skill is even worst than Shiro from Fate. At least that guys only states the obvious. But this dude just keeps speaking nonsense while trying to convince himself that's his quote is cool.