Does anyone know the name of a late 70's / early 80's fantasy movie where a boy is on a boat and he stabs through the floor of the boat and it turns out that the boat is strapped to the head of a giant. The giant then submerges out of the water and begins walking on land. This was the only vague memory I had of the movie as a child and I cannot remember anything else except that it was a disturbing movie for children??

Oh yes.. I think I can nail this one:

Time Bandits (1981) (http://www.imdb.com/title/tt0081633/)

They inject the giant with a sleeping potion... that when it come out of the water and sits down on land to nap...