Mare Respiratus

"Sometimes I think the ocean is a living thing, I know there's something off about it. If I stare long enough, something primal from deep beneath the waves reaches out to try and pull me in... Why else do you think every month the sea rises to such a high point?" -????
The oceans of the planet Althasi (and to a very very minor extent, Raxodin) fluctuate in accordance with the Waking World's current moon phase. Every month, the sea level rises to its peak during a full moon and drains away to nothing during a new moon.
For Althasi specifically, the only means to travel between its floating island cities is when the current moon phase is leaning towards a new moon unless one has the means to ride upon the raging ocean or fly amidst the storms for long periods of time.
The phases are as follows-

First Quater:

The sea level is at its average height, and the flora on the planet is middling as well. One can expect the occasional rain storm, although it is quite tame in comparison to the storms seen during a full moon. The first and last quarters are perhaps the goldilocks zone of Althasi's climate, and travel between cities is relatively easy even if one is provided with a barebones mode of transportation via the air or sea. The floating island cities themselves hang low in the air, preparing to dock with the land for the coming full moon.

Full Moon:

An 'unending storm' has begun, and the sea level has risen well enough to flood the now-docked cities. All of the flora that grows upon Althasi normally has drowned and now lay in heaps underneath the waves. Compare to the drizzles and rains of the first quarter, the power of this storm is comparable to a category 5 hurricane. At present, there are no means to travel between the cities during this phase, as the unruly waves and awful winds dash the attempts of creatures and man-made constructs alike when leaving the city confines. The cities themselves are shielded from the worst of it all, but upon reaching its outskirts does one feel the true magnitude of the full moon's influence.

Last Quarter:

By now, the storms have begun to calm, and the sea level has lowered back to where it was during the first quarter. Some of the flora previously seen attempts to regrow, and the floating island cities undock and begin to rise up once more. The winds are still relatively strong from the storm during the full moon, but nowhere near as hazardous. Rainfall does happen in the first quarter, but very infrequent. Travel between cities can once more be attempted, as long as one has the means to do it by air or sea.

New Moon:

The oceans have drained away completely, exposing the sandy desert-like seabed. All precipitation has ceased, leaving only faint, gentle gusts of wind on the occasion. Once more, all of the flora upon Althasi has died, this time due to lack of water, and their shrivelled corpses dot the exposed land. The floating island cities are unaffected by this drought, in fact, they are now suspended at their highest point in the sky, held to their docks by their various chains. The flooding they sustained during the full moon continues to nourish the flora within the city as well as provide the cities with their own localized weather- thus it is a natural thing to see rain over the city even during the new moon. Travel between cities is now at its safest and is possible on foot or horseback, so long as one can safely descend the cities from their anchors.

Manifestation

The entire sea level, not just the tide, fluctuates according to the waking world's current moon phase. A full moon is when the sea level is the highest, while a new moon leaves the ocean with nary a drop.

Localization

At present, the only two places affected are Althasi and to a very minor extent Raxodin. Raxodin does not experience desertification during new moons, nor massive flooding during full moons.

Type

Metaphysical, Psychic