Since before recorded history - since the first person looked at a floating log and realized... "hey, I can ride that to the other side of this large body of water". Seriously there were likely boats (of a sort) being used as far back as 130,000 years ago in Crete. - http://en.wikipedia.org/wiki/Boat#History
Dugouts are the oldest boats archaeologists have found, dating back about eight thousand years.Boats have served as transportation since early times.
Circumstantial evidence, such as the early settlement of Australia over 40,000 years ago, and findings in Crete dated 130,000 years ago suggests that boats have been used since ancient times. The earliest boats have been predicted to be logboats. The oldest boats to be found by archaeological excavation are logboats from around 7,000-10,000 years ago. The oldest recovered boat in the world is the Pesse canoe; it is a dugout or hollowed tree trunk from a Pinus sylvestris. It was constructed somewhere between 8200 and 7600 BC. This canoe is exhibited in the Drents Museum in Assen, Netherlands; other very old dugout boats have been discovered! A 7,000 year-old seagoing reed boat has been found in Kuwait. Boats were used between 4000 and 3000 BC in Sumer, ancient Egypt