I enjoy how exceedingly arcane some radio "firsts" are. Officially the first submarine broadcast was December 7th 1930. But on October 15th 1919 there was a two-way radio communication between Commander Clark Withers of the US Submarine H-2 and the US Destroyer Blakely. the submarine was submerged in the Hudson River off 96th street NYC. There is almost a decade between those two events. I didn't believe there was that size gap in experimentation.
In 1924 a diver from Philadelphia named C. O. Jackson, broadcast from the bottom of the Atlantic Ocean. A microphone was mounted inside his helmet wrapped in sponges to protect it from impact and to mute his voice at that range. the carbon mics of that era were very delicate.
He did not broadcast from the bottom. A cable ran back up to the boat where a more traditional transmitter and antenna rig relayed the signal to WIP-AM. From there the city of Philadelphia could hear Jackson describe the briny deep. At the time WIP was owned by the Gimbel Brothers Department Store.
WIP began it's first official broadcast on March 18, 1922. WIP had been a flashy station from day one. Back then, as a day share with WGBS in New York, they were audible straight into Manhattan. They broadcast from inside a glass DJ booth so that any onlooker could watch. So broadcasting from the sea floor may have been a stunt, but it was a stunt hundreds of thousands of people heard. More here.