As with all firsts I will first define my terms. By Submarine I mean the craft which travels underwater. I also mean that the broadcast occurred while the craft was fully submerged. A submarine on the surface is no different than a boat, right? This type of transmission was employed throughout WWII by all sides so it would be no surprise that it was developed before the conflict began in Europe. What's surprising is how much earlier.
The Navy took an early interest in radio, they were outfitting their fleet with radios as early as 1916, but only for use at the surface. Some sources I found give the first Submarine broadcast a date of December 7th 1930. These give a location of New London, Connecticut. I think we can all assume they mean off shore. More specifically "they" were 10 miles off shore in Long Island Sound. By "They" I mean that there were in fact two submarines; the USS 0-4 and the USS 0-3. First Lieutenant Norman S. Ives gave a brief description of the mechanics of submarines aboard the USS 0-4 while submerging. Afterward Lieutenant George C. Hern gave a similar talk and also allowed the listeners to hear the acoustics of the submerging USS 0-3. The shortwave signal was picked up and rebroadcast by WEAF, with announcers George Hicks and James Wallington presiding.
The book "Connecticut Firsts" by Wilson Faude cites this 1930 event as the first submarine broadcast as does the "Kane Book of Famous Firsts". The problem with the off-cited date of 1930... is that there is a very credible citation in the Bureau of Engineering Monthly Report of Radio and Sound for an event 10 years earlier. Since this was a prepared broadcast it is obvious that there was testing even technological steps that had to precede it. A photograph of the USS 0-7 here clearly has a radio on board. (pictures) More here.
In the year 1919 the U.S. Navy sponsored a joint project to achieve transmission to and reception by submarines. This project involved the Edison Society, the U.S. Bureau of Standards, the Hammond Laboratory, and the Marconi Wireless Telegraph Company. They tested an array of antenna and transmission types. The most crucial discovery was by Willoughby and Lowell of the Bureau of Standards. It was discovered that the very low frequencies suffered much less attenuation in sea water. Attenuation limits the range of radio signals and is affected by the materials a signal must pass through. We call this Path Loss. (I'll get into that in detail some other day.)
In their culminating test, a U.S. Navy submarine off the coast of New London received a signal on 24 kc that was broadcast from a radio station at Nauen, Germany, 3,234 miles away. Furthermore.. the 1921 Text book "Principles of radio communication" by John Harold Morecroft et al. clearly spelled out proper use of loop antennas on submerged submarines. This leaves two questions.
1. Why do so many texts list the 1930 date?
2. What happened in the intervening 11 years?