1) choose an anfibian or a seaplane, place it in a seaport or on the water, then Shift+J, choose the configuration, then 1 if you want the ship just where you are, or 2 a mile ahead and so on. Choosing 5 you can place the ship in a fix point, (press alt to see the map).
2) choose an aircraft and place it on an airfield close to the sea, take off and fly to the open sea, then again Shift+J, choose the ship, then 1 or2,3. You will see it just 1,4 or 10 miles in front of you. Then again Shift+J and 1 to start its motion, and so on.
Try it, it's not difficult at all. Enjoy it!