Technically Sutton Hoo was a buried ship, not a burial ship.
Let me offer one more technicality. The Sutton Hoo ship was clearly a burial but on dry land, not at sea. It was found in a group of mounds in Suffolk and is considered to be the burial site of an early 7th century Anglo Saxon Wuffing king or other high official. The amazing regalia found with it is on display at the British Museum and is not to be missed if you're ever in London.
Burials in ships are fairly common around the world. Several dry land ship burials have been found in Abydos, Egypt that date to around 3000 B.C. The symbolism of sending the dead off to an unknown paradise (Valhalla or...?) in a ship was probably a pretty universal idea back in the days when the world's geography was largely unknown. It has a lot in common with the current notion of a "heaven" in the unexplored reaches of space.