Alibaug is one of the nearest beaches near Mumbai and Pune which in fact is one of the reasons for its popularity. The easiest and fastest way to reach Alibaug from Mumbai is by the ferry from Gate way of India. There are A/C ferries too that ply between Mumbai and Mandwa which is a boon especially during the summers.
How to Reach - Alibaug..
Alibaug is the head quarters of the Raigad district and is very well connected to other parts of the state by road. The nearest railway station near Alibaug is Pen which is about 30 kms from Alibaug. There are direct trains like 'Ratnagiri Pass' from Dadar and Diva SWV Pass from Panvel to Pen, however if you have to use public transport the ferry service is a far more convenient option.
Mumbai to Mandwa
Mandwa jetty is where the ferries alight from Mumbai. The ferry operators from Gate way also have bus service from Mandwa to Alibaug bus depot. If you are traveling with your two wheeler then you need to go to 'Bhaucha Dhakka'.
If you are traveling by a two wheeler or doing a bicycle tour you can travel the entire distance between Mumbai and Goa with out leaving the coast line. There are five creeks in all between Mumbai and Goa. From Diveagar you can stay on the coastline even if you are traveling by a four wheeler thanks to the Ratnadurga shipping corporation which plies regular ferry service that carry four wheelers between Bagmandala - Bankot, Dabhol - Dhopave and Tavsal - Jaigad.