For what reason would a civilisation experiencing an almost golden age in industry and agriculture, experiencing no catastrophic plague almost all move underwater in as such that they live entirely underwater in similar sized cities as previous. The land they exist on has not changed and is still as suitable as before so for what reason would they make the move.
Edit: I would like to stress the main problem I'm having is the fact that the majority of the civilisation, although not necessarily all, need to have a reason to move underwater rather than just small groups. To bring more context for this: Our civilisation let's call them the Maya, have been prospering and the misery and failures that have long plagued their Empire have been weathering the storm that is the Great Technological Revolution, bringing new advancements that made farming easier, people healthier and allowed cities bigger and grander then ever before accommodating numbers of people that would of once filled the entire Empire ushering in a new golden age of development and prosperity. Yet this Empire chooses to risk it all and gamble on what could either be the extension or end of this era of progress with a disruptive mass migration to the depths of the sea.
To clarify once more this question is not asking the possibility or feasibility of such a move under water, but instead that if such a feat was an option although I must stress not an incredibly easy one, what reasons would force a civilisation to make this change.