Only purified water should be used for drinking, brushing your teeth as well as hand washing. Many, if not most, accommodations provide at least one bottle per guest per day.
According to the CDC tap water is generally not safe for consumption.
According to the Numbeo water drinkability and accessibility index, tap water has a score of 39.63 / 100 (100 being best).
Numbeo considers this score to be "Low"
1 adult could save about 657$ per year in Dominican Republic by drinking tap water instead of buying bottled water.
๐ณ Drinking tap water would also help with reducing your plastic footprint and improve environmental pollution.
Like in most countries, water quality and accessibility differs from location to location, therefore it's the best to check the information available on that place.