Mumbai to Goa is a long journey by road. There are many trains running to and fro between these two at almost all times of a day. Some good trains take 10 hours and by road it's a 12 hours journey.
During monsoons i.e. from June till August the beauty of the Western ghats gets multiplied. Green litchen, moss and red soil along the highway make it picturesque. Due to incessant rain water gets clogged on hills and makes its way down in the form of waterfalls. There are many small water passages along the highway but can not be called waterfall technically.
There are natural waterfalls which don't get dried up post monsoon. To quote 3 best out of all; The one in RANPAT, MARLESHWAR and AMBOLI.
RANPAT waterfall is 3 km on the right from the Mumbai Goa highway. While going from Mumbai to Goa it comes after Chiplun, specifically Sangameshwar. There is a right turn 12 to 15 km after Sangameshwar on the right. Take the side route, the road itself takes to a spot where the trek starts. An uphevelled train bridge can be spotted from that place and a small water stream passes from under the road. Start the trek from there. The foot trails left by other trekkers can be followed to the railway line. Hardly 500 meters. Upon reaching the railway line a tunnel called RANPAT TUNNEL, 843 METERS LONG can be seen on the left. Cross the tunnel, which is safe even if a train passes when u r inside. On the other end of the tunnel, the magnificent waterfall can be spotted on the right. This place is traceable on Google maps also.
Marleshwar waterfall emerge from Shiva Marleshwar temple which is 43 km sideways from the main highway. It is on the Sangameshwar pangri road. While starting from Mumbai side leave the highway 16 kms or so after Sangameshwar towards Devrukh. The road signs are not available on this route. Also GPS works intermittently. Better ask the locals. The vehicles are parked well before the waterfalls. Then 200 to 300 steps need to be taken, which are not steep at all and not tiresome. After the steps there is a Shiva Temple caved into a big rock. The entry is conjusted due to small mouth of the cave. Post temple is the waterfall which appears white and grand from distance. The water falls into a valley of mountains. The tips of the mountains are not visible due to clouds.
AMBOLI waterfall is in Sindhudurg district. Nearest train station is Sawantwadi road. Taxies can be booked for 2000 rupees outside the station. A bit over expensive. It is a 30 km drive from Sawantwadi road station. The small stretch of road journey is very enjoyable and scenic. AMBOLI is very close to Goa. Many tourists visit this place throughout the year.
If you are fond of Hollywood movies where road bends along mountains on one side and sun shining on the sea on the other, do visit Are Vare. This place falls between Ratnagiri and Ganpatipule. 12 km from ratnagiri. Every inch of the route is amazing. The 25 km journey from Ratnagiri to Ganpatipule is covered in 2 hours, because people stop a lot to absorb the scenic views alond the road. Ganpatipule has a famous Ganesha temple on the sea shore. This place is very famous too. Hotels restaurants and market area is also there.