The US Center for Disease Control (CDC) does not recommend drinking tap water on Dominica but many visitors do it without incident. Bottled water, though, is cheap and widely available.
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 0.0 / 100 (100 being best).
Numbeo considers this score to be "Very low"
1 adult could save about 1460$ per year in Dominica 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.