it's all a matter of time. They have one year to move. If a unit takes up half a year to get from land to the coast and onto the ship, then the ship only has half a year left to move. (The first half is wasted, waiting there.)

A ship with troops vs a ship without, if starting from the same place at the same time, will travel the same distance.

This is perfect, and kudos to CA. In other games with fixed movement points, you can get weird things happening.