The reefs and rocky outcrops around Sindhudurg Fort host the most accessible scuba diving on India's west coast. Operators on Tarkarli and Chivla beaches run introductory dives suitable for non-swimmers — a brief shore lesson followed by a guided descent to around eight metres — as well as full PADI courses over several days.
Snorkelling trips by boat are the cheaper alternative and require no booking in advance. The best months are mid-October to mid-May; the monsoon shuts everything down completely. Avoid weekends and holidays unless you actively enjoy crowds.