...And here are the mostly legal early highlights:
1898 East Goodwin Lightship First Marconi tests in the Thames estuary
1902 Nathan Stubblefield broadcasts from the ship the Hartholdi in the Potomac River in Washington, D.C.
1907-09 The Great White Fleet in an attempt to exert a wider international influence, broadcasts to passing ships and to land based communities from a umber of ships: The Alabama, the Connecticut, the Georgia, The Illinois, The Kansas and Kearsarge to name a few.
1919 President Woodrow Wilson makes an Independence Day broadcast on 2380 kHz for the coastal areas of the US from the USS George Washington. More here.
1924 USS Leviathan Broadcasting several concerts from the Atlantic as "WSN"
1933. A scientific vessle, theBathysphere, Callsign ZFB1 broadcast descriptions of underwater seascape to bermuda. It'd be a footnote except that it was carried by the NBC mediumwave network.
1942 A navy vessel the Kron Prinz, was outfitted in the Caribbean with a powerful radio transmitter. The original "Kron Prinz" was allocated the callsign DPZ during the spark wireless era. This is somonewhat incongrouous as there was also a Kron Prinz decommissioned in 1919 and sold for scrap to the Boston Metal Company, yet another record I found indicates the ship was sunk in WWI.