There are two types of Services offered in Indian Navy.

First of all, you can as an Officer and other as a Sailor.

As an officer you have a lot of opportunities in different aspects of Navy like you can be a pilot In the Indian Navy, you can be In the ordinance Branch of Navy.

Now depending upon your Qualification you can join any of them. For most the services you are Eligible if you have a Mathematics Background same goes for Airforce too.

If you want to join as a medical officer then you should choose Biology as a subject. But if you are not interested in Medicines then I would insist you take Mathematics as a steam.

After class 12th You can appear for NDA and select Navy as your preference.

The Mathematics asked in the NDA consist of 11th and 12th class so you will get an advantage over there.