It's not wise to drink tap water if you're only in Turkey on a short visit. Stick to bottled water, boil tap water for 10 minutes or use purification tablets or a filter.
Do not drink river or lake water, which may lead to diarrhoea or vomiting.
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 52.31 / 100 (100 being best).
Numbeo considers this score to be "Moderate"
1 adult could save about 204$ per year in Turkey 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.