From the Martyrology:
At Cracow in Poland, St. Hyacinth, confessor, of the Order of Preachers. Having received the religious habit from the hands of our Father St. Dominic, he excelled in learning and in a life of admirable innocence. He was celebrated for the glory of his miracles, especially for walking dryshod across wide rivers. Thought deserving of sweet converse with the holy Mother of God, distinguished for his spotless life, and filled with the gifts of the Holy Ghost, he died at an advanced age. He was called to his eternal reward on the very feastday of the Assumption of the Blessed Virgin Mary. He was canonized by Pope Clement VIII.
From the Office of Matins (Lesson iii):
Hyacinth was born of noble and Christian parents at the castle of Camin in the diocese of Warsaw in Poland. Being appointed one of the canons of Cracow, he surpassed the others in piety and learning. At Rome he was received into the Order of Preachers by the founder himself. To the end of his life he religiously adhered to the perfect way of life that he had learned from Saint Dominic and also maintained perpetual virginity. On being sent back to his own country, he established six houses of the Order. It is almost incredible what progress he made with all by preaching the word of God, and by the innocence of his life. He was renowned for many miracles. Particularly striking was the miracle of his crossing the Vistula, without a boat, when the river was in flood near Visograde, taking his companions also with him on his cloak which he had spread upon the waters. Having continued his remarkable way of life for almost forty years after his profession, he returned his sould to God on the feast of the Assumption of the Blessed Virgin in the year 1257. He was canonized by Clement VIII.
Prayer
O God, you made the blessed Hyacinth, you confessor, glorious among the peoples of different nations by the holiness of his life and the splendor of his miracles; grant that by his example we may amend our lives, and by his help be defended in adversity. Through our Lord…