Rabbi Chaim Breisch, aged 59 according to police, was rescued with another man after struggling in a rough sea off Broadstairs but was pronounced dead on arrival after being taken by helicopter to hospital.
A Satmar Chasid, Rabbi Breisch was the former head of Mesivta, one of the most established yeshivot in the North London district.
Marton Braun, a friend, said he was "an outstanding scholar from one of the most eminent scholarly families, a perfect gentleman".
Chanoch Kesselman, the executive co-ordinator of the Union of Orthodox Hebrew Congregations, said he was "highly respected, the whole community is shocked".
Police said emergency services were called on Monday morning after reports of two men in difficulty.
The second man, according to the website Voz Is Neias, was Yitzchak Beigel – Rabbi Breisch's Talmud study partner. Rabbi Breisch was staying at his brother-in-law's holiday home.
Some have speculated that both men entered the water for their daily ritual immersion (in lieu of a local mikveh), according to the custom of the most pious Chasidim.
An inquest into cause of death has been opened and adjourned.