I'll explain what I understood.
There should be two worlds, so the one kraam enter through the gate in the last ep should be a parallel world, where Joey is still alive. Even in the 6th episode arisa and the two other go through the gate and entre a parallel world where joey is still alive. In Kraam's world the tsunami affected one way and in the other it affected in another way as described in the poem.
But in my explanation there is a plot hole, if Arisa and the 2 other went through the cave to the night of the party, why did it reverse back after sometime? I guess the explanation to this is that they didn't go through the proper gate which Kraam used, they might have used a temporary one because they didn't go underwater to cross the gate like Kraam did.
It is a parallel world but it's a little different from Kraam's world. There are 2 options, it should either be a time travel or parallel worlds, but in given circumstances it should be a parallel world.
Lol idk if my explanation is plausible :P
And I have another doubt, idk if i didnt pay attention in the drama but why did the student's parents didn't bother to help the students? Or is it like the parents tried to find them but the students were stuck in an unreachable place like in another world? lol so many questions running in my mind rn