I do enjoy the game a great deal but I have some ideas that might allow some improvements. It makes no sense that it takes years for a boat to go from Alaska to Chile. People can move about any place around the world in a year baded on supplies and their knowledge of land ability to navigate.
Take sea units and base them at some city and they have a rang they can do missions, transport units, engage in commerce or explore. Sea units can act more like air units. When they do a mission at war time they can be intercepted then do combat. There should be an option to put a fleet in an enemy port if it is not defended and bombard the enemy or conduct siege.
The explorer is a good idea. There can also be scouts that can move and live off the land but larger formations should start from a base an have a limited range or have some logistics such as fleets or other supply units that must be built to keep them in the field. It can make sieges interesting in that the sieging army has to be supplies.
The owner can build supply and move them. Any other civilization should be able to build supples and sell them or there should allow for pillage to stay supplied but that causes resentment and possible partizan's resistants or terrorism.