Tarkarli is in Malvan, about 550 kms from Mumbai off the Mumbai - Goa highway. A great place to stop for a day or two before you get into Goa. Driving time to Goa would be about an hour if one takes the inside route that reaches you to Sawantvadi from where you can take a 5 minute ferry ride (including the car) into main Goa or take the slightly roundabout route by road.
Tarkarli is one of the most beautiful beaches along the konkan coast with its pristine white sand. To stay in the MTDC resort is the best bet being sea facing. It is fairly economical too at about 1500 per night for an AC cottage. It is best advised however to drive into the village for meals for the taste of some authentic coastal Maharastrian seafood fare.
A boat ride that last between 2-3 hours in the backwaters can be taken in the morning to go dolphin spotting. Its one of the few beach places left that is not overflowing with tourists and can be a great place for a romantic getaway with a terrific view. People looking for just some place to vegetate or chill out can stay longer, ones who need some activity to keep the adrenalin going after this should not stop for more than 2 days.
I understand one can also rent a houseboat to stay on the backwaters but have not personally rented one to be able to recommend it or otherwise.