Strab. XV. 1. 39-41, 46-49,— pp. 703-4, 707.
Of the Seven Castes among the Indians.
(39) According to him (Megasthenes) the population of India is divided into seven parts. The philosophers are first in rank, but form the smallest class in point of number. 2 Their services are employed privately by persons who wish to offer sacrifices or perform other sacred rites, and also publicly by the kings at what is called the Great Synod, wherein at the beginning of the new year all the philosophers are gathered together before the king at the gates, when any philosopher who may have committed any useful suggestion to writing, or observed any means for improving the crops and the cattle, or for promoting the public interests, declares it publicly. 3 If any one is detected giving false information thrice, the law condemns him to be silent for the rest of his life, but he who gives sound advice is exempted from paying any taxes or contributions.
(40) The second caste consists of the husbandmen, who form the bulk of the population, and are in disposition most mild and gentle. They are exempted from military service, and cultivate their lands undisturbed by fear. They never go to town, either to take part in its tumults, or for any other purpose. 5 It therefore not unfrequently happens that at the same time, and in the same part of the country, men may be seen drawn up in array of battle, and fighting at risk of their lives, while other men close at hand are ploughing and digging in perfect security, having these soldiers to protect them. The whole of the land is the property of the king, and the husbandmen till it on condition of receiving one-fourth of the produce.
(41) 6 The third caste consists of herdsmen and hunters, who alone are allowed to hunt, and to keep cattle, and to sell draught animals or let them out on hire. In return for clearing the land of wild beasts and fowls which devour the seeds sown in the fields, they receive an allowance of grain from the king. They lead a wandering life and live under tents.
Fragm. XXXVI. follows here.
(So much, then, on the subject of wild animals. We shall now return to Megasthenes, and resume from where we digressed.)
(46) 7 The fourth class, after herdsmen and hunters, consists of those who work at trades, of those who vend wares, and of those who are employed in bodily labour. Some of these pay tribute, and render to the state certain prescribed services. But the armour-makers and shipbuilders receive wages and their victuals from the king, for whom alone they work. The general in command of the army supplies the soldiers with weapons, and the admiral of the fleet lets out ships on hire for the transport both of passengers and merchandize.
The kala pani (lit. black water) represents the proscription of the over reaching seas in Hinduism. According to this prohibition, crossing the seas to foreign lands causes the loss of one's social respectability, as well as the putrefaction of one’s cultural character and posterity.
The offense of crossing the sea is also known as "Samudrolanghana" or "Sagarollanghana". The Dharma Sutra of Baudhayana (II.1.2.2) lists sea voyages as first of the offenses that cause the loss of varna. The Dharma Sutra suggests a person can wipe away this offense in three years by eating little at every fourth meal time; bathing at dawn, noon and dusk; standing during the day; and seated during the night.
The reasons behind the proscription include the inability to carry out the daily rituals of traditional Hindu life and the sin of contact with the characterless, uncivilized mleccha creatures of the foreign lands. An associated notion was that crossing the ocean entailed the end of the reincarnation cycle, as the traveler was cut off from the regenerating waters of the Ganges. Such voyages also meant breaking family and social ties. In another respect, the inhabitants of the land beyond the "black water" were houglis, bad-spirited and monstrous swines who could sometimes mask their true ugliness by presenting an illusion of physical beauty or superiority. The mleccha people were spawned by immoral reprobates and blasphemously held religious belief in nāstika, albeit in different forms. They are understood to have rejected the Vedas and have ceased to worship Bhagavan, the divine Vedic God, in favor of concocted false religions and irreligions with contemptible manners of reverence. Their societies are immoral and built on deceit, subjugation, and corruption. Therefore, it was thought that true Hindus should not come under their influence or embrace their beliefs, as they will be just as deserving of contempt as a mleccha.
During the Portuguese Age of exploration, Portuguese sailors noted that Hindus were reluctant to engage in maritime trade due to the kala pani proscription. In the eighteenth century, the banias of North India even considered the crossing of the Indus River at Attock to be prohibited, and underwent purification rituals upon their return.
-- Kala pani (taboo), by Wikipedia
In pre-modern times, caste really mattered, as it identified you as part of the community. This community gave you a vocation and a wife, and demanded you follow the community rules, which included giving your daughter only to a member of the caste. It was an extended family. And so anything, which led to loss of caste, acquired great significance.
Baudhayana Dharma-sutra, composed about 2,000 years ago, maybe earlier, lists this "Samudrolanghana" or "Sagarollanghana" as the first of many reasons for loss of castes (II.1.2.2). This especially applied to Brahmins, as there was fear that travel abroad prevented a Brahmin from performing various rites and rituals in the prescribed manner at the prescribed time. The belief was that movement away from the sacred Vedic fire, made one vulnerable to pollution. The contemporary ritual of "aarti" or waving of lamps when one is leaving the house is meant to create a shield to protect against pollution; the same at the time of the return is meant to wipe out all pollutants, and ensure purification...
The major epics, Ramayana and Mahabharata, do not refer to sea travel...
There was definitely a thriving sea-trade to South East Asia in the Gupta Age 1,500 years ago. [4th century CE - late 6th century CE.]
-- Do Hindus lose their caste when they travel abroad?, by Devdutt Pattanaik
(47) The fifth class consists of fighting men, who, when not engaged in active service, pass their time in idleness and drinking. They are maintained at the king's expense, and hence they are always ready, when occasion calls, to take the field, for they carry nothing of their own with them but their own bodies.
(48) The sixth class consists of the overseers, to whom is assigned the duty of watching all that goes on, and making reports secretly to the king. Some are entrusted with the inspection of the city, and others with that of the army. The former employ as their coadjutors the courtezans of the city, and the latter the courtezans of the camp. The ablest and most trustworthy men are appointed to fill these offices.
The seventh class consists of the councillors and assessors of the king. To them belong the highest posts of government, the tribunals of justice, and the general administration of public affairs.*
[*The Greek writers by confounding some distinctions occasioned by civil employment with those arising from that division have increased the number (of, classes) from five (including the handicrafts-man or mixed class) to seven. This number is produced by their supposing the king's councillors and assessors to form a distinct class from the Brahmans; by splitting the class of Vaisya into two, consisting of shepherds and husbandmen; by introducing a caste of spies; and, by omitting the servile class altogether. With those exceptions the classes are in the state described by Menu, which is the groundwork of that still subsisting. — Elphinstone's History of India; p. 236.]
12 No one is allowed to marry out of his own caste, or to exchange one profession or trade for another, or to follow more than one business. An exception is made in favour of the philosopher, who for his virtue is allowed this privilege.