You have to apply at least 4 days in advance of your date of arrival to India as the visa holds a validity of 30 days.  Apply here: www.ivisa.com


Thus if you apply say on 1 January, then you can enter India any time during the 5th of January to 4th February as the approval takes approx. 3 days. 


Then the visa hold 2 (double) entries and you can ONLY enter India through these 16 designated Airports: 

Ahmedabad, Amritsar, Bagdogra, Bengaluru, Calicut, Chennai, Chandigarh,Cochin, Coimbatore, Delhi, Gaya, Goa, Guwahati, Hyderabad, Jaipur, Kolkata, Lucknow, Mangalore, Mumbai, Nagpur, Pune, Tiruchirapalli, Trivandrum & Varanasi. 

And 3 seaports: Cochin, Goa and Mangalore.
You can however leave the country from any border post.  You can stay a maximum of 60 days inside India!

PS: If you arrive by boat to another seaport, then you have to contact the cruise company or other service provider for more details regarding the correct type of visa you have to get.