Minmaster is correct, the difference between sea and ocean tiles probably accounts for your sinking events.
You can use your right button from your mouse to click on sea or ocean tiles to display the terrain info and make sure what they are. The pathfinder (pointing to move circle) algoritm also helps you with this process because it will display a number in the forward circle when you can legitimiately move into the forward square.
Remember also that the great lighthouse adds one movement point to the base movement range of all your naval units.
Another big issue to not forget is that the great lighthouse will give you the power to cross sea square and the extra movement range to even cross some ocean gaps that are under 4 tiles wide but you will not have the ability to pass resources across teh gaps until you discover Astronomy or perhaps even Magnetism (or Navigation).
Good luck,