Lost!

[bewitched]

Originally Posted by BarTopDancer

Add to that, at the start of the season the island was underwater.

So, we have

An island that is now underwater after an H-bomb was set off (presuming it actually went off and they didn't 'jump' right before it exploded).

People who have been living on the [now] underwater island still living on the above ground island and somewhere else.

I think the island is only underwater if/when the sideways timeline is allowed to exist. Further, they aren't living on the island and somewhere else concurrently (at least based on given information). The sideways timeline is 3 years earlier than the island timeline which may or may not have ever existed to begin with (if the sideways timeline is allowed to continue).

Did that make sense?