This question is addressed to Saint Thomas Christians that are Catholic. Since the Catholic church rejects the authenticity of both the Gospel of Thomas and the Acts of Thomas, how can anyone in the Catholic segment of that faith say with confidence that Thomas sailed from the Red Sea to the port city of Muziris in 52 AD?
Both the Gospel of Thomas and the Acts of Thomas are considered apocrypha by the Catholic Church and as such are not deemed to be part of the deposit of faith.
No one can say with absolute confidence that St Thomas sailed to Muziris, India in 52 AD. Nevertheless both tradition and legend in this domain are believed by many of St Thomas Christians. Their belief in this matter should be considered more of a popular belief than a absolute truth.
What we do know is "that about the year A.D. 46 a king was reigning over that part of Asia south of Himalayas now represented by Afghanistan, Baluchistan, the Punjab, and Sind, who bore the name Gondophernes or Guduphara. This we know both from the discovery of coins, some of the Parthian type with Greek legends, others of the Indian types with the legends in an Indian dialect in Kharoshthi characters. Despite sundry minor variations the identity of the name with the Gundafor of the "Acta Thomae" is unmistakable and is hardly disputed." - Catholic Encyclopedia
"Though the tradition that St. Thomas preached in "India" was widely spread in both East and West and is to be found in such writers as Ephraem Syrus, Ambrose, Paulinus, Jerome, and, later Gregory of Tours and others, still it is difficult to discover any adequate support for the long-accepted belief that St. Thomas pushed his missionary journeys as far south as Mylapore, not far from Madras, and there suffered martyrdom. In that region is still to be found a granite bas-relief cross with a Pahlavi (ancient Persian) inscription dating from the seventh century, and the tradition that it was here that St. Thomas laid down his life is locally very strong. Certain it is also that on the Malabar or west coast of southern India a body of Christians still exists using a form of Syriac for its liturgical language. Whether this Church dates from the time of St. Thomas the Apostle (there was a Syro-Chaldean bishop John "from India and Persia" who assisted at the Council of Nicea in 325) or whether the Gospel was first preached there in 345 owing to the Persian persecution under Shapur (or Sapor), or whether the Syrian missionaries who accompanied a certain Thomas Cana penetrated to the Malabar coast about the year 745 seems difficult to determine." - Catholic Encyclopedia
Acts of Thomas is the earliest extant reference to Thomas’ journey to India and conversions of Indians. Keith Hopkins says in A World Full of Gods, page 154, the Acts of Thomas was written in Syriac, probably at the beginning of the third century in Edessa, Syria.
R. E. Frykenberg says in 'India', published in A World History of Christianity, page 152, it is clear that Christian and Jewish communities were already settled along the shores of the subcontinent from the second century onwards. Chronologically, this makes it possible for the author of Acts of Thomas to have been aware of a Christian community in India and to have elaborated on, or even invented, any tradition that the community was founded by Thomas. On page 150, Frykenberg calls the Acts of Thomas a romantic tale of questionable historicity.
Frykenberg says (page 148) that local traditions have indicated that the Apostle Thomas came by sea from Arabia and landed on the Malabar coast. Alternatively, they have shown that he came overland, down from the north. Details of arrival and subsequent events have been celebrated in song and verse for generations untold. The existence of two slightly different traditions about the arrival of Saint Thomas give cause for concern about the historicity of both, as does the fact that the traditions only existed in song and verse for so many centuries.
India's biggest archaeological dig is being undertaken at Pattanam, thought to be the ancient port city of Muziris, where the main tradition says that Thomas landed in India. No evidence of Thomas has yet been found, but the faithful remain confident that success will come in time. At this stage, the story of St. Thomas sailing to Muziris is best regarded as a legend.
Interestingly, a large number of people who accept the belief that St. Thomas preached in Kerala, India are non-Christians while the skeptics, sadly, are mostly Christians of various denominations and rites. For instance, the Travancore State Manual (Published in 1940) at Vol I Chapter-X Religion-Christianity (page 713) discusses the issue at length and reaches at the conclusion that St. Thomas did preach in Kerala and built up a community of Christians there. Whether he had reached Kerala via sea or land route, should of little significance given that he preached the Gospel.
For more information and details, see the resources at the Kerala State Central Library.