I'm not sure it changes all that much, to be honest.
While breathing underwater will be very useful, it doesn't change a lot of other things that make water a less ideal habitat for humans. For instance, the human body is naturally positively buoyant. This means her inherently float in water. This isn't just a matter of "air in the lungs" thing, this is an entire body thing. Remember: corpses of people who've drowned still naturally float.
This means that actually living underwater would be a constant exercise in frustration for humans. You'd have to weight yourself down, you couldn't just leave a baby in a crib to sleep, but rather would have to construct cages. Plus the human form, while surprisingly good at swimming, isn't naturally oriented towards it, walking is much more efficient, and swimming requires all four limbs, whereas walking leave two hands free to do other things.
This does allow considerably more efficient exploitation of near shore resources and also makes crossing rivers much easier for humans. That said, it has very little impact on deep sea or ocean going, as the issues there are less humans drowning and more straight up logistics and the sheer physical challenges of dealing with waves and the like. See, the thing is, boats were never about preventing drowning. Seriously, they're not, they've about transporting supplies with humans, either resources taken FROM the water or taking resources with humans ACROSS water. That use isn't going to really change even with the gills. After all, you need some place to put the fish or other resources you get from the water to store them before going home, or you need something to keep all your stuff together in as you cross the water.
However, this would change how humans interact with water. Water would be less dangerous to humans, thus we'd treat it, in some way, less seriously. You might, might see some sort of primitive aquatic nomadic tribes in certain regions where the water stays warm and calm most of the year, but this would really alter, say, northern European interactions with water all that much. Sure, they can BREATH the water, but that's not going to change the REAL danger of getting wet in northern Europe: hypothermia.