Boat - ship
This is an area where there is different usage in different social groups. In ordinary speech, a passenger vessel like a cruise liner is often called by those who travel on her ‘the boat’. The professionals who staff her more usually call her a ship, as do her builders, insurers and so on. In the days before air travel became common, in the first half of the twentieth century, passengers across the Atlantic referred to the boat, and indeed went from London to Southampton very often on the boat train. The crew, meanwhile, worked on the ship.
Those who are interested in the old days of sail will insist that a ship is a vessel with three or more masts, carrying square sails on all her masts. There are wonderful possibilities for pedantry in this particular sphere of knowledge! (See also Bark - barge - barque. In the modern world of powered vessels, a good rule of thumb might be simply that a ship is big enough to travel in the great oceans.
The Royal Navy has warships and (rowing boats). Some classes of armed vessel however are known as boats, particularly the submarine, which in its role as a carrier of nuclear weapons is the most dangerous of all.