And following on from your point, I suppose this is relevant as well: channel or estuary perhaps? (makes you wonder where the English eight you mentioned for a narrow bit of water comes from - haven’t heard that one?)
gŵyth = faen; ffrwd, cornant, ffos, (geir.) cwter, carthffos; sianel; braich o fôr yn ymestyn i’r tir, aber: vein, sinew, nerve; vein, seam (in min.); stream, brook, ditch, (dict.) gutter, drain; channel; firth, estuary
Also in Cornish Gwyth means trees and lots of houses called things like Chy an Gwyth etc in Cornwall
Wikipedia does suggest celtic associations for the island as its possible root, but also says no-one really knows - actually a very long and important celtic past to this little place - changed hands several time. Wheat was traded/used here 8000 years ago - you could also pull carts across the solent at low tide at this time.
I suspect that there may have been words spawned in this area - associated with trade over a long period time, from the mesolithic, through the iron-age to the Romans, that have filtered into a lot of different languages.