From my knowledge:
Vehicles, people, and boats USED TO always move at the same speed they were when they first left dock/town/whatever. So if it were calculated that you move at 10 'units' an hour (Just go with me here), then you would also go at that speed.
However, it has been changed. Or so I've been informed. Moving speeds are now recalculated when the turn comes to move... you... yeah.
So, if you had eaten food, thrown a body overboard, or given some resources to someone not in your vehicle, then your speed will increase. If you get damaged, begin to starve, whatever - your speed will decrease.
This of course is just from information that I have been told in the past, and from experimentation within game. I could always be wrong, since I seem to be quite often
