Since I often play huge islands maps, ships have a high priority for me in the early-mid game. I usually skip caravels altogether due to their fragility and go directly for Galleons once the economy has stabilized enough to support 3-4 of them (one of which takes the extended sight perk). They then sail around as a dispersed group concentrating forces for wiping out sea monsters and taking their loot and, in case of war, quickly returning to do some very rewarding shore bombardment of cities or exposed troops and one-shotting enemy cogs.
Enchanted high-quality land units eventually end up superseding them for these tasks, but Galleons are available earlier in the game (two buildings in any city with coast) and can kickstart your progress. Those Sea Serpent and Kraken lairs are nice. Unless you start with nearby high-cost resources to exploit and an AI enemy you want to rush, why not produce a few Galleons and get a headstart on the mid and end-game spells as well as regular huge cash and mana rewards?
I find going for a handful of galleons in the early midgame quite valuable on continents maps too for the same reasons, though to a lesser degree, as I'll more often end up pouring all my resources into early land wars. Even then, mostly it pays off to go scouring the seas with galleons as early as possible.
