I think "on the sea" is kind of implied by the fact that it's a ship... most people don't get on a ship in a drydock, right?
They can get on a ship on a river or lake, too. Remember that Hungary is a landlocked country, so for Hungarians the sea may not spring to mind quite so readily ... :-)
Boat, ship... If you have only one word in hungarian both should be accepted
Hey, don't undermine csónak. That's really a boat, a small one. Rowboat, mostly. And there's vitorlás which is a sailboat.
In my dictionary boat is for csónak and hajó can be a ship (if it is bigger) or a boat (if it is smaller)