[Prince Charon]

One thing that GURPS Banestorm, and 3e GURPS Fantasy before it, does that most of the other fantasy settings I've seen for GURPS do not, is to make mana levels geopolitically important (as seen in the map on Banestorm p176). The border between Caithness and Meglos is defined by the line along which normal mana becomes low mana, and their border with al-Wazif is affected by it. Agartha (see the GURPS Banestorm supplement 'Banestorm: Abydos') is scary because the mana there is death-aspected, and the folk who live there use that to their advantage. The Ring Islands are interesting and dangerous specifically because of their mana levels, the Great Desert is feared in large part because of its utter lack of mana, and so on. Psi normally has no equivalent to this, though (which I think was a factor in my taking so long to even start working on this idea, despite having it years ago). Here are a few ideas on how to handle that:

The first is to simply declare that psi in this setting is affected by something like mana levels, perhaps Sanctity, or a new term like 'Resonance' or 'the Planetary Psi Field.' This psi field might sometimes be aspected to certain powers, subcategories of powers, or specific abilities or uses (e. g. 'psychokinetic manipulation of formerly-living objects'). It seems to me like a lazy option, but it does preserve the geopolitics of the setting as written.

A variant of the above is to have a form of 'anti-psionic radiation,' left over from the Banestorm summoning and/or ancient wars, that exists in parts of Ytarria. It's strong in the Great Desert, effectively neutralizing psi abilites with few or no exceptions (depending on whether you want to allow some psionic equivalent of Mana Enhancer to exist in-setting). The radiation is weaker in Caithness and the areas around it (as well as a few other spots on the continent), giving penalties around -5 to active use of psi skills, and is itself neutralized in a few parts of Caithness by certain meteors. This doesn't help explain the 'low mana at sea' issue, though, and requires some sort of 'pro-psi radiation' in the Ring Islands and other spots of high or very high mana. It also doesn't help explain the variable mana areas.

Perhaps the animals, plants, and other lifeforms in certain regions have meta-psionic abilities, as well as other psi powers, and or some parts of Ytarria might have an abundance of psi-reactive crystals. Large numbers of meta-psionic plants can account for even the variable-mana regions, though the Great Desert seems more like a good place for anti-psionic crystals. Common anti-psionic deep-sea life, or some from of plankton that rarely drifts toward shore (or something like that) explains the difficulty of using powers at sea. These natural resources would be of great interest to many, whether to collect and cultivate for use in elixirs and psychotronic devices, or to control the psychic population, or even to destroy for personal, religious, or political reasons. This version would somewhat alter the culture seen in the books, but would likewise be a useful source of story-hooks.

An interesting option, though one requiring a fair bit of work from the GM, is to just not have an equivalent to mana levels at all, instead coming up with non-psi-related reasons for all geopolitical and other concerns. Perhaps there is a wide and/or well-fortified river or canal on the border between Meglos and Caithness, for example, or the land, being close to the Great Desert, might be less fertile, so Meglos isn't as interested in controlling it. The Ring Islands having lots of exotic, non-psionic, but poisonous and/or venomous plants and animals is quite reasonable, and the Great Desert just being an unusually dry and hostile desert (like Death Valley, but larger) likewise works fairly well.

You might also combine a few of the above, such as the Great Desert being full of anti-psionic radiation, but the Ring Islands being home to a great diversity of interesting psionic and non-psionic life.