What are your thoughts about the Caravels? Even though they can be improved by the Great Lighthouse and the Grand Canal (as most of other naval units, though), I almost never build them. I think the Explorer is its worst nightmare: both can explore the ocean at the same time, but only the Explorer earns experience doing so; both have the same cost; one more tech (Astronomy) and they have the same move in the ocean; the caravel can explore only the oceans and the coasts, the explorer can explore (sorry) everything.
Overall, the caravel seems to me a very niche unit for maps full of water and coastal cities, and civs willing to conquer them with a navy, while you can use your explorers in any map to discover other continents or islands and raise their level until they become very valuable units, and worth of upgrading without any doubts.
Maybe increase the visual range of the Caravels by 1 and reduce the Explorers' to 1 while embarked?