For a 3 days trip i would suggest you these 3 places which is convenient and nearby palces according to your location. 1. Alibaug (145kms)
Its famous for water sports and is replete with sandy beaches and pleanty of forts too. 2. Tarkarli (400 kms)
Famous for its long and narrow stretch of beaches with pristine water along with scuba diving as one of its famous water sports. 3. Kolad (145kms)
known for its cascading waterfalls where you can enjoy rafting, Kayaking and rappelling.
I think Gokarna would be a good option but I am not sure about water adventures. However, you can have some trekking and camping adventure at some of the beaches and on some hills around Om beach.
Depends on how many people you're, I would suggest to have a group of people to camp because I have heard it is not too safe to camp alone or as a couple on top of the hill. You may pitch your tent at the beach. That would be safer.
Murdeshwar and Gokarna. Murdeshwar will take only a day to visit. you can visit the Shiva temple and chill at a resort with a beach view. You can then head to Gokarna by train which is on two hours. Gokarna is a very chilled out place and has a great vibe.