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According to this article in Hinduism Today (July/August/September 2008), samudrayana (ocean voyage) is forbidden in the Shastras, but it may not be binding on current followers -- instead they may go through ritual purification after travel. The relevant passage is below:
The Baudhayana Sutra, one of the Hindu Dharma Shastras, says that "making voyages by sea" (II.1.2.2) is an offense which will cause pataniya, loss of caste. It offers a rather difficult penance: "They shall eat every fourth mealtime a little food, bathe at the time of the three libations (morning, noon and evening), passing the day standing and the night sitting. After the lapse of three years, they throw off their guilt."
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Raja Ram Mohan Roy was the first hindu Brahmin who ignored the old rule of ocean travelling prohibition... on 15th November 1830.
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Samudrayana, overseas travel, has always been forbidden to an observant Hindu because it would necessarily involve coming into contact with a non Hindu, which is an uncleansable defilement. In itself, there is nothing wrong with travel, but to have contact with foreigners, with the Dasyu, and their food is a fundamental violation of Hindu Brama, purity of being.
In Hinduism, everything is gradation. Thus, a Brahman who defiles himself by martial activities becomes a Ksatriya, which in itself has its own honors, or perhaps something worse. The more defilement you bring upon yourself, the deeper your dishonor and degradation. This is what is meant by "loss of caste". Conversely, by right and pure conduct a person may ascend and become higher in grade, even a Brahman.
To have regular contact with unbelieving foreigners, the Dasyu, is considered to be so defiling that there is no coming back in one lifetime.
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The term in Sanskrit "Sagara Ullanghana" or "Samudra Ullanghana" is the term mainly used to prohibit upper caste i.e. Brahmins who have learnt Vedas and do daily 'Pujas' and 'Sandhyavandanam' from crossing the sea or ocean. This article Hindus and Ocean Taboo gives the complete picture of it and also what a Brahmin says about "making voyages by sea".
Baudhayana Dharma and Grahya Shastra and Manu Smriti extensively mentions castes and imposes strict rules to be followed by those castes. But most of them concentrate on Brahmins and their Do's and Don'ts. But samudrayana was allowed to other castes because since Vedic period people from India have traveled across the world for trade. And another example is Hindu culture in Indonesia.