I’m much more familiar with the Southeast Asian (SEA) side of things than the India side of things, but I hope this will be sufficient to shed some light.

The restriction you’re referring to is known as ‘kala pani’ (black water), and it wasn’t a blanket, unchanging rule. Rather, it was viewed differently by different Hindus depending on moment in time, caste, vocation, community and variety of Hinduism. The number of Hindus who actually refused to travel overseas because of it seems to have been rather small, and it does not seem to have prevented large numbers of Hindus from crossing the ocean in search of gold and glory.

The earliest known mention of a ban on crossing the ocean is in the 5th century BC sutra of Baudhayana, which also contains a purification ritual to remove this ‘sin’. In the Laws of Manu, a text that outlines the norms of domestic, religious and social life from around 500 BC, it is also stated that a brahmin who undertakes a sea voyage is considered a sinner. However, the text does not state that such a person would become untouchable.

The restriction in these texts seems to apply only to Brahmins. S. C. Bindra (2002) has postulated that other castes may have adopted it as a way of emulating the Brahminic practices.

It is difficult to say whether there would have been more or less of a seafaring tradition on the Indian subcontinent had the kala pani taboo not existed. However, we can definitely say that Hindus, especially from southern India, travelled widely despite this apparent restriction.

Take, for instance, Hindu merchants, who have a long history in SEA. There is strong evidence of Indian trading activity in Kedah dating from the 4th century.

Beginning from the 9th century we also have Javanese inscriptions that imply significant Indian merchant communities in several settlements and ports. For example, an inscription found in Kedu in central Java contains a list of foreign merchants organised by place of origin. The listed South Asians come from Kling (Kalinga), Aryya (Aryapura) and Pandikira (in Karnataka), implying that the Indian traders were numerous enough to be classified by region, instead of being lumped together under an umbrella term like ‘Indians’.

There are also trade-related inscriptions that date to the 11th century, which is around the time of the Chola invasion of Srivijaya (1025). Take, for instance, this one, from 1088 (I am unable to type the font necessary for an accurate rendering of the Tamil terms, apologies if some of the nuances are lost):

In the Saka year 1010 current, month Masi, we, the Nanadesa-Tisaiyayirattu Ainnurruvar [a merchant association active in south India and Sri Lanka], having met at the velapuram [settlement on the seashore] in Varosu [present day Barus], also called 'the pattinam [commercial town] for the welfare of the merchant body blessed by Siva', decided to grant as follows to 'our sons', the nagara-senapati [commander of the commercial settlement] Nattu-cettiyar [commander’s name], to Patinen-bumi-desi-appar [meaning unclear], and to the mavettugal [elephant-trainers?]: [unclear] ships' [unclear, possibly cargoes?], the ship's captain and crew will pay the fee anju-tundayam [possibly a reference to the ship’s size] in gold, pegged to the price of kasturi [musk], and [unclear] may ‘step on the cloth spread’ [enter the settlement to trade]. Thus we, the Five Hundred of the Thousand Directions, known in every direction in all Eighteen Lands, had the stone inscribed and planted. Do not forget charity; charity alone will help you.

The Chinese court also records the arrival of a Chola trade mission in 1015, and the according of first class status to the Chola state.

So Indian merchants undoubtedly crossed the seas. But what about Brahmins, to whom the original kala pani restriction supposedly applied? There is also ample evidence of their travels.

For example, there were numerous SEA Hindu-Buddhist kingdoms from the 5th to the 15th centuries. These kingdoms were ruled by locals, but had adopted many aspects of Indian political structure and religion. It was once thought that these practices were brought by Indian traders, however it is now believed that it was Brahmins, with their familiarity of religion and statecraft, who brought this knowledge to SEA.

The Indian merchants also built temples to service their communities. The remains of one such example can be found in Quanzhou, China, along with a Tamil inscription dating to 1281:

Obeisance to Hara [Shiva]! Let there be prosperity! On the day Citra in the month of Chittira in the Saka year 1203 [1281 A.D.], the Tavachchakkarvarttigal Sambandhap-perumal [a Shaiva religious leader] caused, in accordance with the firman [written permission] of Chekachai Khan [the Mongol authority], to be graciously installed the God Udaiyar Tirukkadalisvaram Udaiya-nayinar [Shiva], for the welfare of the illustrious body of the illustrious Chekachai Khan.

But Hindu temples do not function by themselves, and must have been staffed by Brahmin priests from India, especially when they were first built.

Thus, the reason the Cholas launched an invasion of Srivijaya despite the kala pani was probably that the kala pani was not much of a restriction for the vast majority of the Empire. There probably were some Brahmins who really did refuse to cross the ocean, but nowhere near enough to hamper an invasion.

So why is it now assumed that the kala pani was so much more serious than it actually was?

(Continued in reply)