A naval cover, according to the Naval Cover Museum, is "any envelope, postcard, or other postal medium that is mailed from or somehow related to a navy ship, location, or event. Beginning in 1908, post offices were established on board U.S. Navy ships and each ship had one or more postmarks to "cancel" the stamps used on the cover. The postmark, or cancellation, would usually have the ship's name and the date that the cover was cancelled."
This cover, postmarked May 13, 1933, Navy Yard, Boston, MA, is from the U.S.S. Du Pont (DD-152). Named for Rear Admiral Samuel Francis Du Pont (yes, he was one of those Du Ponts), the destroyer was launched in 1918 and (apparently these are different things) commissioned in 1919. At the time this cover was mailed, the Du Pont was operating out of Boston as a training vessel. With the outbreak of World War II, she was recommissioned (for the second or third time) and became part of the Neutrality Patrol; eventually, she escorted convoys across the Atlantic and was one of the ships that brought casualties back from Normandy. She was decommissioned for the last time in 1946 and sold in 1947.
When I get old covers, I like to see what I can find out about the addressee--in this case Ralph R. Spiker of New Philadelphia, OH. It seems that he was a collector of naval covers--several have turned up on eBay and other auction sites. He died in 1982.
The remaining question here: why the frog cachet? I can't discover any connection to the U.S.S. Du Pont, no frogs in insignia or anything like that. My guess is that it's an add-on. Oh, well. It's still an awesome frog.
A few weeks after I posted the first cover, I found this one. It's postmarked March 12, 1935 on the U.S.S. Heron. Commissioned in 1918 for minesweeping, she worked in the North Sea, clearing mines laid during the First World War. Later, she performed similar duties in the Pacific. At the time this cover was cancelled, the Heron was a tender ship for seaplanes in the Asiatic Fleet. She received the Navy Unit Commendation for her actions during the Pearl Harbor attack. She continued to serve as a tender throughout the War, and was transferred to the State Department "for disposal" in 1947.
I suppose the illustration--the blue heron, if not the frog--is self-explanatory.
The addressee, one Denver J. Todd, turns up in an alumni bulletin of Kalamazoo College (class of '36), where he is listed as an agent-operator for Pere-Marquette, and in this family tree. He died in 1986.