I will answer as if everyone here is in an "environmental" mindset.
#1, tapwater comes to homes in Santo Domingo from a water plant, where, theoretically, when it leaves the plant going into the tubes, the water meets international potable standards. The contamination comes into play in the distribution system with pipe leaks and breaks.
This SHOULD preclude any nervous Nelly worrying about PCB and heavy metal contamination in the potable water supply, but not eliminate the worry about bacterial/viral/parasitical contaminations.
#2, you don't have to BOIL water to make it safe from bacterial/viral/parasitical contamination. You can simply PASTEURIZE the water, which is much more environmentally friendly, as it only requires about half of the energy to do so. Here's one source of information:
A SUMMARY OF WATER PASTEURIZATION TECHNIQUES
#3, after you've been here in the DR for a couple of years, your body should have adjusted to the different bugs/germs/whatever that would give a visitor the turkey trots. I don't use bottled water, and I never have this problem anymore. I do take anti-parasite medicine about every 6 months as a precaution, but that's it, and I cannot remember the last time I had any bathroom problems.