I have an Indian visa valid till 2013 in my old passport. But since I just got married I'm changing my surname and hence changing my passport.
Will my Indian visa in my now old passport be valid, provided I have all Marriage documents?
Yes it is valid, as long as you will provide both passports and the marriage documents. The visa was given to you not to your name.
Why I am sure of this? As an airline employee I have seen many of my colleagues changing their names because of marriage and they still be able to use different visas from different countries with no problems at all.
Travisa, who were until very recently the official source for obtaining Indian Visas in the US have the following in their FAQ :
Question: My name has changed or is changing; do I need a new visa?
A. If you have not obtained a new passport, and you still have your non-expired passport with your prior name and a valid visa issued in your prior name, please travel with your marriage certificate or proof of name change.
B. If you have been issued a new passport in your new name and you still have your previously issued passport (with a valid visa), please request transfer of visa and submit a copy of your marriage certificate or proof of name change.
C. If you do not have your previously issued passport, please apply for a new visa with your new passport.
Whilst this does not say that you can not travel with your marriage certificate and both passports, it at least implies that this is not recommended.
BLS, who have replaced Travisa for Indian Visas in the US, no longer have this question in their FAQ, but they do suggest applying for a transfer even if your name hasn't changed.
In addition to what MeNoTalk said, depending on the country you are from, if they just cancel your old passport and send it back to you, you have to make sure that the visa itself on the old passport is not cancelled or annotated with anything other than what is supposed to be there.
I don't think you have to travel with a marriage document assuming you had to produce it anyway before you got an updated passport. This is a fairly common situation and as long as you can produce both the passports, you shall be fine.