Waterworld
If the planet in question had low land mass compared to ocean, then they may simply have run out of land to live on relative to population size/growth. Or there may be land, but formed in small, widely separated continents which are inconvenient for trade. Creating trade outposts in the ocean because they can, could provide lucrative opportunities for those who can afford it. Eventually, trade centers become rich and a population draw on their own merits.
Alien Encounter
Divers may encounter a sentient species living in the deep ocean, and wish to move closer for research and trade. Or, it could be that the continents are, for anomalous reasons, metal-poor (at least in the metals they care most about), but the oceanic crust is metal-rich. Or oil. Or valuable farming fish. Economics, as stated by all.
Expansion, Not Migration
If the land is suitable for habitation, then it does not make sense that they will leave it. Rather, it only makes sense that they would expand away from it. All species fall into the Malthusian Trap, which says that they reproduce to the subsistence level of their habitat. 258
Another way to say it is that all species expand to fill every ecological niche that they can, humans being no exception. If you want people to leave the land, then you need to have a reason they can't/won't stay. Crop failure, dust storms, earthquakes, dangerous predators, etc. are all good reasons to leave while they can.
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Religion
If, for some reason, the priestly caste decided that living in the ocean would be preferable, they just need a story to tell why the gods have deemed it necessary, and the appropriate inducements to sway doubters who might prefer to stay. A prophecy about a coming catastrophe on land (asteroid, fire, floods, etc.) would help. Note that a similar prediction could be made by scientists, but if it is bogus, you would need a police state to ensure that none of the honest ones spill the beans (or at least, are believed).
Megalomania
If the empire is strongly authoritarian and led by an effective dictator, then you simply need a reason for said person to want to migrate. For the glory of the motherland! To prove that we can! For the glory of me!!! Because I have foreseen a glorious future under the sea!
Space Exploration
If the Maya are also entering the Age of Rocketry, they may have designs on colonizing space. Building under water presents many challenges similar to building on a moon or an orbital habitat. Especially if there is a time sensitive component involved, like an anticipated mega-caldera that could bring on an extended volcanic ash winter and destroy all structures on the planet's surface. Then, building underwater would force citizens to quickly develop technology for making structures air-tight, safe, redundant, closed, etc. 222