r0se-of-bl4des

I don't have the Forgotten Sea Myth, so I'll leave this YouTube video here for reference:

Here is how it describes the Sea God Ceremony:

No one knows what happened that day. Lemuria is plunged into endless darkness. Panic seeps into every part of the Ocean. The Deep Sea is angry. In its wrath, waves slam against the cliffs and rocks fall into the waters. Even that under-built temple is turned into rubble. 'Tis a long time before the Sea God walks out of the temple alone, bringing light back to Lemuria. As he leaves the temple, the alcove that once held the flame collapses. The Sea God's follower is gone. Bathed in sparks and cinders, he holds a small, pulsing flame. It writhes like a newborn. Some said the flame was born from the purest faith. Some said the Sea God sacrificed his follower to save Lemuria... Different stories, some true, some not, circulated until people gradually forgot about the ceremony. No Lemurians have ever seen the Sea God's follower and the little fish that swims around him - they are gone, whereabouts unknown. Only many years later than the Sea God has perished, when Lemuria has sunk to the depths of the Ocean, a rumor begins to circulate Lemuria: The God of the Sea had lied to the Deep Sea. For his beloved. Lemuria: The Days to Come

From this, we know that the God of Tides' actions did not destroy Lemuria. Lemuria had brief scare and then continued on as normal until he "died". Tears of Romirro goes into further detail, telling us what happened from his and MC's perspective:

So, he never intended for Lemuria to fall. He knew that Lemuria would survive even if the Ceremony was not completed that day and was actively looking for a means to keep Lemuria safe and whole that did not depend on the ritualistic murder of innocents based on a racial stereotype.

Unfortunately, the legends he had heard about proved to be a myth. And MC was forced to lock him away to extend his lifespan.

Let me point you back to this line:

Only many years later than the Sea God has perished, when Lemuria has sunk to the depths of the Ocean, a rumor begins to circulate Lemuria...

This is likely when Lemuria's first "destruction" occured. After Rafayel was locked away, Lemuria sank deep into the ocean and most of its people fell into a slumber, aside from a small few, only one of which Rafayel and MC knew of:

...and he killed that guy. So he didn't have a people to protect or sacrifice, only himself.

The God of Tides sacrificed his power, and the Sea God sacrificed his life, but neither did anything to harm Lemuria.

What about Linkon City Rafayel?

The only indication we have that he brought about modern Lemuria's downfall is this passage of "Siren's Ballad (Pt. 5)":

I'm inclined to believe "the young boy" is Rafayel because it immediately cuts to him having a flashback to Lemuria's destruction. The waves staining the shore red could be carrying his blood, as Detective Louis's story in the same anecdote suggests that he had his scales cut the same day that Lemuria was destroyed.

At this point in time, I have seen no evidence pointing to the incident having anything to do with helping MC, so he can't be said to have sacrificed Lemuria for her here. It's more likely that he innocently gave up Lemuria's location believing the sailors were friendly, only for them to show their appreciation by cutting off his scales and leaving him to die.

(Of course there is an alternate interpretation: The sailors already knew where Lemuria was and happened to encounter him while on their way, meaning his homeland's destruction had nothing to do with him at all.)

And Abysswalker?

From what we know currently, Lemurians have been surviving the desert for 30 millennia at this point.

They don't need the sea. Drowning the people of Philos might bring them comfort, but they have proven that it is not necessary for their survival.

What he's done is make Lemurians responsible for their own fates without interference from the Tome of the Sea God, at the cost of his own life and happy ending. In his eyes, his actions mean MC and Lemuria will live on but he will not.