Ships

Ships can be used to transport cargo by sea. However, they are slow. Because of this slowness, they may have negative income before delivering goods. On the other hand, they don't require lots of infrastructure, but there are extra that may cost you.

Strategies
Ships definitely can't go long distances, as they are very slow. Ships can help deliver oil to oil refineries along the coast. You should make sure that the dock is near the refinery. If you can't get close to it (the land won't let you), you can transfer the oil onto the station by selecting 'transfer' in the orders screen. You can then use road vehicles or trains to pick up the oil and take it to the refinery.

If trying to go into an inland sea/lake, you can use canals or terraforming the land. Canals can be efficient when trying to go around cities/industries. Terraforming makes "natural" pathways if the ground is brought down to sea level. If you have lots of money, you could create an artificial lake by digging a hole and then letting water into it. You can add some docks and create a transport system in the lake you created.

Pros

 * Is the only vehicle that can travel on water
 * Can access Oil Fields offshore
 * Can deliver Goods, Oil, and Passengers

Cons

 * Mostly slow
 * Requires additional infrastructure if they want to go over land without terraforming the land
 * May end up in negative due to slower speed

Canal/Canal Locks
Canals are a way for ships to pass through land, and connect inland lakes to. In order for a canal to be connected to an ocean, you need canal locks. Canal locks take up a 1X3 space, and must be on a slope in the middle (see image). Ships will move slower in a lock, but will return to normal speed in the canal. Try to not mess up, as removing either will cost a lot of money.