Somewhere down the line maybe. I'd also prefer some UI work and some general polish first. 

Yes to boats, but I'd also like to see a number of features added/improved upon beforehand.

Features like more polished, realistic, and interesting battles -- with legitimate obstacles that actually block projectiles, objects that can hurt a unit if knocked into them, ranged weapons that actually have a limited range of usefulness, varied elevation with cliffs that are actually dangerous etc.  After that, incorporate legitimate dungeons, more creative options like designing your own palace, bring back dynasties with far more options on raising your children (deciding who they will become thus enhancing their stats, more options for betrothal or simply 'selling them off' etc).

...and perhaps after all THAT, I'd consider implementing coastal buildings and naval warfare.

They've got boats in some of the pre-made maps.  

I'd much prefer some all land maps, water is currently nothing more than a border.

I liked having the boat in the scenario but it was too slow.

Boats could start off being just transports but with combat between them being between the units they carry.  Boats without units would have no defense or attack.

Boats could be built when a city has a dock developed.   Allow cities at a beach to develop piers etc.   Allow pier when a city grows next to a river or beach.

Civ V (GnK) turned the boats into "melee" units. It seems to work ok that way. Boats are fine as long as the AI knows how to use them otherwise it's too cheesy.