The Sea of Barnacles is a sea in the world of Daventry.
Background[]
It is located three thousand miles west of the Vale of Obscurity, west of the Hibestian Range. Though it’s not clear if this means it lies on the other side of the world beyond the western sea near the Crimson Sea, or instead closer to west coast of a very large continent where Daventry is located (Serenia/Daventry).
The regions near the sea are home of a species of poisonous toad. The Ice Fairies moved to the coast of the sea after the Vale of Obscurity was destroyed. It must be a region of cold and ice for the fairies to live comfortably there.
The sea lies somewhere west of the western edge of the continent of Daventry, within or further west beyond the Western Sea. It appears to be coastal sea lying along a large stretch of land, perhaps a continent or large island, or lying between more than one large islands (similar to the Irish Sea). However the latter is less likely as it is suggested to have a single coastline. Perhaps it lies beyond the Northern Sea into the Unknown far to the west near the Arctic.
Behind the scenes[]
If it is a reference to the western coast of Daventry, then it is giving the continent of Daventry a much different scale than in the games or King's Quest Companion.
In the games and Companion the continent and space between continents are treated as lying within a day or two of each other, and the continent itself being a few hundred miles across at the most (accounting for the fact that land's are very small in the games), and characters get around them so 'fast'. For example it took Alexander a day or less to travel between Llewdor and Daventry in KQ3, and a day or less to make it to the Kingdom of Daventry proper (Rosella had been tied up over the space of two days).
Even Kolyma is considered to be 'nearby' Daventry in KQ2 showing a much smaller scale to the world. However some sources suggest Kolyma is “far away” from Daventry suggesting inconsistent scales.
The 'western sea' is mentioned in both King's Quest Companion, and Kingdom of Sorrow. So at least the western sea and Sea of Barnacles at least do not appear to be the same location. The reference in See No Weevil does seem to suggest that the western sea is at least close enough to western edge of Daventry for the winds to blow in. See no weevil, also speaks of at least part of Daventry being to the south, near the sea (aka Southern Sea in the Companion, creating further scale questions).
The fact that the Ice Fairies appear to need to live in places of cold, it would seem that Sea of Barnacles is likely Arctic in nature. Which may suggest its actually of a northwest trajectory. Which could place it beyond the Unknown. The Northwest portions of the continent of Daventry would not fit, as that region contains the hot Endless Desert (and at the time of the story also largely taking up much of the northern edge of the continent where remaining half of Serenia was located as shown in Wizard and the Princess). The Great Mountains themselves (the section which Alexander crossed in KQ3) or near Icebella's realm would likely be cold enough, but not anywhere near three thousand miles away from the Hibestian Range.
It is not likely to be located on Tanalore either, as that continent does not seem to be cold enough either except maybe high up in impossible mountains (filled largely with humid swamps, and hot deserts in the middle, and temperate lands on the coasts).
The description of it being on a coast would seem to suggest that it is next to its own continent, and less likely islands.