A root for “pool, mere, lake” appearing in etymological notes from 1957 (PE17/145, 160), and also appearing as ᴹ√LIN “pool” in The Etymologies of the 1930s (Ety/LIN¹). In both instances it was the second element in Q. ailin “(large) lake”, and so connected to S. ael “lake” (N. oel) as suggested by Christopher Tolkien (SA/lin¹). In the 1957 notes Tolkien said the root √LIN had a “Sindarin differentiation > glin-”, but I can find no indication of this in any attested words.
In The Etymologies of the 1930s it was connected to ᴹ√LINKWI with derivatives ᴹQ. linqe/N. lhimp “wet” and N. lhimmid “moisten” (Ety/LINKWI; EtyAC/LINKWI). This is turn was probably a later iteration of the early root ᴱ√LIQI “flow, water; clear, transparent” with derivatives like ᴱQ. linqe “water”, ᴱQ. liqin(a) “wet” and ᴱQ. liqis(tea) “transparence (transparent)” (QL/54). For purposes of Neo-Eldarin, I think it is best to assume the “wetness” senses were transferred to ᴹ√LINKWI, but I think it is worth positing a Neo-Eldarin root ✱ᴺ√LIKWIS “clear, transparent” to preserve words associated with tranparency.
This root may be associated with √LIN “make a musical sound”; see that entry for details.