No multi-megaton nuclear warhead ever has been exploded underwater, so the best we can hope for in the way of sources is "what is the research on this".
Wikipedia: Underwater explosion -- Deep underwater explosion:
An example of a deep underwater explosion is the Wahoo test, which was carried out in 1958 as part of Operation Hardtack IOperation Hardtack I. A 9 kt Mk-7Mk-7 was detonated at a depth of 500 ft (150 m) in deep water. There was little evidence of a fireball. The spray dome rose to a height of 900 ft (270 m). Gas from the bubble broke through the spray dome to form jets which shot out in all directions and reached heights of up to 1,700 ft (520 m). The base surge at its maximum size was 2.5 mi (4.0 km) in diameter and 1,000 ft (300 m) high.[6]
During the Cold War, underwater explosions were thought to operate under the same principles as tsunamis, potentially increasing dramatically in height as they move over shallow water, and flooding the land beyond the shoreline.[7] Later research and analysis suggested that water waves generated by explosions were different from those generated by tsunamis and landslides. Méhauté et al. conclude in their 1996 overview Water Waves Generated by Underwater Explosion that the surface waves from even a very large offshore undersea explosion would expend most of their energy on the continental shelf, resulting in coastal flooding no worse than that from a bad storm.[2]
The Operation Wigwam test in 1955 occurred at a depth of 2,000 ft (610 m), the deepest detonation of any nuclear device.
Source [7]: Glasstone, Samuel; Dolan, Philip (1977). "Shock effects of surface and subsurface bursts". The effects of nuclear weapons, (third ed.). Washington: U.S. Department of Defense; Energy Research and Development Administration.
Source [2]: Le Méhauté, Bernard; Wang, Shen (1995). Water waves generated by underwater explosion. World Scientific Publishing. ISBN 981-02-2083-9.