John Rhys, in his study Celtic Foklore (1901), discussing a poem in The Black Book of Carmarthen , dealing with the inundation of Cantre’r Gwaelod on the shores of Cardigan Bay, says this:
“The name … Mererid, Margarita, ‘a pearl’ … but what does it here mean? …. the name given to some negligent guardian of a fairy well. It cannot very well be, however, the name belonging to the original form of the legend.”
My own meditation on this question follows, based on my experience of this landscape and its resonant features.
The remains of trees on the beach at Borth, Cardigan Bay
What is the name of the well maiden?
– the one who wept
tears of grief when the seal was broken
so the engulfing waters swept
over the land , drowning the forest that watched the sea?Was it ‘Pearl’
– a hidden bud
of moisture in the enclosing earth
and stone, its pulse swelling to a flood
rushing down the cairn-strewn hill?If not Mererid,
then to what hidden name will she answer
to those who seek the source?
Is she kin to Sulis or Coventina,
or to some sea nymph, say MorganaDwelling now in Gwyddno’s fort out under
the crashing waves
where the old road runs into the sea
her hair laved by the ebb and flow of the tides,
her wail echoed in the seabird’s plaintive cries.
awenydd.cymru 
I used to live in Borth and often walked amonst the ‘trees’ on the shore at low tide . Thank you for a beautiful article