My source tells me that R. Hershel Schachter has reversed his opinion regarding water in New York. I have a copy of his in-house letter on the subject but have been instructed not to post it verbatim. What R. Schachter says is that if you can see a speck in water and the speck is moving (i.e. from that movement you can tell that it is a bug) then that is sufficient to render it visible to the eye as a bug. As proof, R. Schachter cites Rashi, Eruvin 28a sv. tzir’ah:
sheretz is from the language of shoretz (crawls) – something that moves on the ground but is not visible due to its small size, except through its crawling and stirring
This would effectively prohibit unfiltered water. My source tells me that R. Yisrael Belsky is still ruling leniently on the water issue. But make sure to ask your local rabbi.