sea crossing or voyages - samudra yana - kala pani causes outcast in to 6th catogory

this is well explained as per Krishna yuzrvedha - Taittiriya samhitha - Baudhayana sutras part2 - 1.2.2

various offences which removes cast are given and  penance is prescribed.

in that crossing sea is one which makes one loose cast and fall in to 6th catogory after chandala.

here this is also explained this way

Govinda explains samudrasaṃyānam, 'making voyages by sea,' by 'voyaging by means of ships to another continent (dvīpa).'

so I think for the people who travel by sea may not be outcasted but that is not said in those days as there was no chance to fly in air crossing the sea.

penance

eat every fourth meal-time bath three times a day stand in the moarning sit in the night for 3 years to get back their cast.

other cast people can not get the brahmin cast this easily by doing this, but only go to their respective cast. 

https://www.hinduwebsite.com/sacredscripts/hinduism/dharma/baudh2.asp#2.2

33. It is declared in the Veda that, on touching a dog, a Kândâla, or an outcast, he becomes at once pure, if he bathes, dressed in his clothes. 33

34. If (while reciting the Veda) they hear noises made by outcasts or Kândâlas, they shall sit silent and fasting during three days;

35. Or if they repeat that (text of the Gâyatrî) at least one thousand times, they become pure; thus it is stated in the Veda.

45. Property received from outcasts, after forming

alliances with them either by (teaching) the Veda

(and by sacrificing) or by marriage, must be relinquished. Let him not associate with such (men).

46. It is declared in the Veda that (he who has

associated with outcasts) becomes pure by reciting

the Sawhita (of his Veda), proceeding in a northerly

direction and fasting.

47. They quote also (a verse) to this (effect), 'A

sinner is liberated from guilt by tormenting his body,

by austerities, and by reciting the Veda ; he becomes

also free by bestowing gifts. That has been declared

in the Veda.'

https://shaivamfiles.fra1.cdn.digitaloceanspaces.com/english/sen-dharma-sutras-vol-2-vasishta-and-baudhayana.pdf

If one associates with an outcast for one year, he himself becomes an outcast; not by sacrificing for him, or teaching him, or forming a matrimonial alliance with him,—but by walking, sitting or eating.

On having intention ally done any one of those acts which cause loss of caste, one should perform a Sāntapana-Kṛcchra; and the Prājāpatya, when it is done unintentionally.

The twice-born, who is performing the Prājāpatya, shall eat in the morning for three days, then in the evening for three days, then for three days food got unasked, and for the next three days he shall not eat. 

penance for outcast given as per  Baudhayana sutras

38. (Eating) during three days in the morning only, during the (next) three days in the evening only, (subsisting) during (another) three days (on) food given unasked, and fasting during three days, (that is) a Kṛcchra penance.^[26]

39. (If the period of twelve days is divided into) three (periods of) four days, that is the Kṛcchra penance of, women, children, and aged men.

it is so much relief for woman and old woman. 

https://www.kinchit.org/dharma-sandeha/thread/can-we-cross-sea-to-go-rameshwaram/

https://www.youtube.com/watch?v=5ea73LYxTPk

he says we should not cross