Okay, but still the question is how Maia got to Landen. I can't imagine that she went through water pipes either. Since you already mentioned, she couldn't have survive it. Or at least her mind had been damaged a lot more than just suffering from amnesia. Simply because of the lack of oxygen.
One clue could be that the sunken temple where Dark Force is trapped seems to be very close to the shores where Rhys found Maia. If the cultists in LaShute had some kind of magical influence it would be plausible for me that this influence could also exist near Dark Force's prison.
So here is my weird and wild speculation: The magical 1.000 years are almost over. DFs powers are growing. Rulakir's and Dark Force's people possess at least one keystone, the stone which opens the passage from Terminus to Aridia. From Aridia they can travel without a stone to Aquatica. They capture Maia and knock her out. Later, they use their powers to teleport her in some way to the sunken palace.
Here things get difficult: Why should they do this and why to Maia? Maybe Rulakir wants to sacrifice her as a high Layan princess to DF but she can escape. She stays a long time underwater and her memories get hurt, resulting in temporary amnesia. Or: The worshippers transport to Landen in order to enable Rhys to find her. In the English translation (okay, not a good source, i know

) they mention something like "You fools did everything we wanted" and "We needed Rhys to bring back the moons." So I guess it could have been planned by them that Rhys found her. But if this was their plan then it would be a really really far-fetched one.
Sorry for this weird ideas. Just some strange theories on a very unlikely event that happened in the game. I simply believe that their is a connection between LaShute's schemes, DF's sunken palace and Maia found on the shores close to the palace.

And now my brain is twisting...