Geoffrey of Monmouth tells us that Arthur built, or assembled, a fleet of ships. 

All three Dumnonian sub-kings Geraint, March or Mark, and Theoderic were not only among Arthur's captains but were also noted seafarers. 

Triad 14 'Three Seafarers of the Island of Britain.' 

From earliest times, the peoples of the south- west have been dependant on the sea, for food, for trade and for defence. So it is perhaps not surprising that in the Welsh triads when we are given the names of 'Three Seafarers of the Island of Britain' two of them were rulers in south-west Britain; Geraint son of Erbin and March son of Meirchiawn,

"Geraint son of Erbin,

and Gwenwynwyn son of Naf,

and March son of Meirchiawn.

Geraint is traditionally linked with the site of the present day village of Gerrans on the south-west coast of Cornwall. A few miles to the East is Castle Dore, a site traditionally linked to King Mark ( March ). The famous 'Tristram Stone' stands nearby.

During this period there was continuing trade with the Mediterranean, with continental Europe, with Ireland; all needing protection; as well as defending the coast against invasion.

The Irish (Scots), the Saxons and the Picts, all the main enemies of the Britons, are recorded over and over as attacking by sea.  

In the south-west with its dependence on the sea the battles may have been fought on land, but frequently, one side arrived by sea.