Why did Indians not venture into the sea (Kala pani)? Was it the lack of marine technology or the attitude that 'sea is dangerous'? Or were Indians not so 'adventurous' so as to take the risk and venture into the sea?

 I recently came across the story of Zarmanochegas - a story of an Indian who immolated himself in Athens 2000 years ago to prove his faith. There is still a tomb of him. His is one story among many Indians who travelled the world, spreading their culture and trade [besides setting up martial arts schools like what Bodhidharma did in China]. This is how the Indian faith of Buddhism spread all over the old world, influencing a variety of world religions [Buddhist influences on Christianity].

Again, Indian artefacts from Indus Valley are all over Egypt and Mesopotamia. How did it get there? And how did Indians settle Australia if they were afraid of the seas? [An Antipodean Raj & Genomes link aboriginal Australians to Indians]

If Indians didn't venture out into the sea, how come the names of many cities and countries in South East Asia Indian? Singapore, Sumatra, Cambodia, Java, Sri Lanka, Laos are all Sanskrit names. How come the kings of Thailand are named after Ramayana characters? How come Indian religions like Hinduism and Buddhism got spread all over that region?

The largest of Hindu temples are in Cambodia - across the ocean.

Here is Hinduism of Bali:

Here is Ayuttya of Thailand, modeled after Ayodhya of Uttar Pradesh in India.

 

 

About 4000 years ago, a group of Indians started the first colonization of Australia. The Harappans at that point were trading with every soul they could find anywhere in the old world.
Genomes link aboriginal Australians to Indians
An Antipodean Raj

Dravidian kings have forever been colonizing Maldives, Sri Lanka and a lot of South East Asia. In the meanwhile, Indian traders and other kings were sailing all over South East Asia influencing the cultures of Cambodia, Bali, Malaysia, Thailand, Laos, Burma, etc.
Greater India: Tracing the Sanskrit Influence in Southeast Asia.

Now that we have established that Indians have been seafarers & influencing cultures in rest of Asia, we will see why India didn't fully convert this into a military advantage.

 

Why didn't India do even further?

The ocean around the subcontinent is massive. Beyond Sri Lanka and Maldives, there was nothing close. Traveling to Africa and South East Asia took a lot of effort. Unlike the Mediterranean where there were a lot of civilizations closely around a peaceful sea, Indian Ocean is monstrous. In the effort it took to get to the other side of the Bay of Bengal or Arabian Sea, you could cross 10 different cultures in Europe.

By the time much more advanced ocean faring ships were invented, Indian civilization had declined and enslaved by outsiders.

Map of Europe drawn to the same scale.

Compare Mediterranean to the big blue sea in the first image.

No major coastal towns because of the brutal oceans

If you see the Indian map, one that that strikes you is that there are no really coastal capitals in ancient Bharat. For a nation with thousands of miles of coast, we have preferred to build our cities and capitals inland. One possible reason is the storms & cyclones that frequently devastate the coast. The few major ancient coastal towns such as Dwarka, Kaveri Poompatinam and Mamallapuram were either taken by the sea or deserted. Even the most seafaring kings - Cholas - built their capital 100+ kilometres interior. Since you couldn't even keep a capital close to the sea, it was not that possible to centre your power around the navy.

Ruled by people from land-locked cultures: North India was always ruled by people from the interior lands, whose rulers were not really exposed to the oceans. The power centres were in interior states such as the UP, Bihar, MP and Punjab. Mauryans, Guptans and Mughals were all land people. South Indian kings on the other hand were sea-farers (grown close to the sea) and much more forceful with their navies.

Lack of need: In the ancient times, Indians felt they had everything - being the leaders in food, textile production and material wealth (gold, diamonds). This complacency didn't create enough urgency to create a navy. On the other hand, Britain, Japan and other island cultures were much more pressed to look outward. Until the Europeans came, there were no other culture that could take a sea-led fight to India and thus the kings were very complacent with maintaining a navy.