What does "strategic location" mean? As in, "India has a strategic location, as it has many sea routes?"

A strategic location is one that would be beneficial to strategy or offer many opportunities for implementing strategy. That being said, "strategy" alone is somewhat ambiguous. In the context of the setting you've provided, it sounds like India is a strategic location for economic pursuits, as waterways can easily be used to transport goods. In this way, India's many waterways would be beneficial in trade strategy and allow for goods or even people—tourists, workers—to get around faster than on land. If a particular company wanted to implement some strategy (like ensuring a shipment of goods within a set time frame), waterways might be beneficial because they do not require the kind of upkeep that roads and railways do.
An alternate meaning to consider is the benefit to military strategy. Waterways create natural boundaries, with sea routes being important ways of getting across or around such boundaries. If the people of India wanted to cut off access to their land or water for some reason, blocking access by sea routes would be a great way to do this.
https://www.dictionary.com/browse/strategic?s=t

https://www.india.gov.in/

https://www.worldatlas.com/webimage/countrys/asia/india/inland.htm