The best months to visit Ratnagiri are November through February, when the weather is dry, temperatures are in the mid-twenties, and the sea is calm enough for the beaches. March and April are hot and dry.
April to June is mango season — the Alphonso orchards around Ratnagiri are at their peak, and coming specifically for the mangoes is not an eccentric reason to visit. The local market fills with crates of Hapus, and you can buy directly from growers.
The monsoon (June to September) brings heavy rain and rough seas. Road travel can be slow and some coastal spots are inaccessible, but the Sahyadri hills are dramatically green and the waterfalls inland are at their fullest. This is not a bad time to visit if you like rain and emptiness — you will often have the beaches entirely to yourself.