J.Glyn Davies in his introduction to Cerddi Huw Puw has a section titled Shanties And Sea Songs on Welsh Ships. There he says: "I have never heard a shanty with Welsh words; most of those I heard on Welsh ships appear, with varying degrees of differences, in Captain Whall's collection. I do not know of any Welsh sea songs of the type that bear that title in Captain Whall's book. There used to be plenty of ballads about shipwrecks round the Welsh coast...It was always the ballad singers wares...".
(If you'd like a copy of the intro - 4 pages - pm me an email address and I'll send you a scan of the full article).