Tthe tap water is not considered suitable for consumption.
There's a great website where you can get an answer on drinking water for any country: canidrinkthewater.org - which is a really useful tool for all travellers.
Safe bottled drinking water is available in DR, just make sure the seal isn't broken. No need for your own purification systems unless you're ethically opposed to bottled water or planning on going hiking and drinking local water.
Phil almost 8 years ago