This is still a work in progress mind you, i have a lot of fleshing out to do.

I'm guessing the sea is a big part of pretty much everyone's cultural background, considering the migrational diaspora

Naval transport and traffic is a pretty big deal, yeah, almost every culture is well versed in maritime affairs. The seas themselves are also windy, making travel often speedy, but making storms harsher and deadlier.

How is your folk magic structured? Is there any other more formalized magic? What do the different magic users think of each other's type of magic?

Another work in progress, but it basicaly works as a set of ritualistic practices, that depend a great deal, upon boddily markings, likes tattoos, to enhance the effects.

Various tattoos and amulets allow people access to various rituals, for healing, growing food and ensuring good and healthy harvests, protecting against accidents, protecting the health of your animals and households.

For example, the most common use of the folk magic in the East and South of Avreth, is to ensure plentiful, speedy and healthy harvests in order to freely feed the population, essentially bumping up food production to our modern level. It bascialy works by saying specific words while consecrating the soil with a special fertiliser. The tattoos help a great deal.

However in the West, folk magic exists purely and solely among a small and wealthy elite who charge for their services, thus reducing most of the population into serfs or other forms of labour. The health and wellbeing of the entire kingdom depends upon this upper class who mercilessly abuse and safeguard the magic by fearmongering that unrestricted use will lead to bad things.