Bromwich and Evans' suggest Operlinnliuan is synonymous with the Llynn Lliwan mentioned in "Culhwch and Olwen" [*line/note:1179]), and, indeed, it appears to be an Old Welsh spelling of it (See Welsh Forms).
They also suggest another form found is Llynn Llyw [note for line 891]. This they hold as derived from lliw "colour, hue, tint", or as an adjective "bright, shining". However, elsewhere [note for line 255] they suggest that lliant, meaning "flood, current, sea", may have been used to name a character associated with the Severn bore and this would seem more pertinent to the lake name as well - note the modern Welsh Lli or Llif (pronounced "Lthiv"?) for "flood".
It should, however, be noted that there is a Ffynnon L(l)uan and ruined Chapel Llanlluan just outside Penrhiwgoch, north of Swansea**. While locals know this ruin as "Chapel of the Owls"*, apparently believing the name derives from tylluan ("owl": plural tylluanod), Thomas* suggests it probably reflects a, now forgotten, Saint Lluan who could plausibly have given their name to the inlet as well. This is not to say the saint wasn't named from one of the terms above.
There is actually a minor celtic saint, Llonio, who may be connected with drowning. Henken gives the following translation of a confused verse about him*...
They were trying to prohibit Llonio from a mansion through hatred, something which they could not do. The man would drown, the way was wicked, as I know, enemy of Maelgwn formerly.
...though it seems a pretty big stretch to leap from this the name Liuan. An alternative is that the name of Llanlluan is somehow related to the name of the nearby Roman Leucarum.
Either way, the estuaries of Carmarthen Bay, which is near Llanlluan, might provide an alternative searching ground for this wonder: either it may be possible that the lake name is related to the name of the major inlet of Loughor/Llwchwr, and in turn this may be related to Leucarum as well*, or the lack of other chapels with the name Llanlluan might suggest a very localized cult for the hypothetical Saint Lluan in the Carmarthen Bay area (though Celtic saints aren't generally the stay-at-home type, and there's no other reason for supposing they should have just stuck round Carmarthen). However, for a Carmarthen location, one would have to argue the unlikely suggestion that Sabrina in the original covered the coast all the way round to Swansea and was, therefore, only loosely coterminous with the "Severn", and that the bore wasn't under discussion.