Ron,
The reason they are called reverse gears is that early gear boxes were mostly direct drive and the basic reason for them was the reverse gear. Later those that were were called reduction gears. I'm guessing at the time frame but prop ably in the 30s to the early 50s the boat gear boxes were called reverse gears. They were usually shifted directly by a lever connected to the gear box.
The expression "tranny" is hot rod slang and has no maritime implications. I consider those who use the word tranny as only loosely connected (if at all) to marine mechanics.
Gee whiz Eric, I respectfully submit the following: Your phrase "maritime implications" poses the implication that TF is filled with just life-long mariners whose life is dependent upon the sea and nothing more. When actually most are simply pleasure boaters who indulge on other life activities too. Calling boat’s "transmission" a reverse gear, trany, trans, trannie, gearbox or any other nomenclature is really not the point of this thread... the different names used by different boaters all refer to the similar mechanical device. Figuring what fluid is actually the correct ones to place inside any of our various brand and model "gear boxes" is.