You realize Hawaii is made up of mountains, correct?

Depending on what the threat is, underwater could offer an additional layer of protection?

> all this money

All what money? Scaled for a net worth of 100k, this would cost less than $200.

Plot twist: the Hawaii bunker is just the decoy bunker

In general it is better to act as those who tell you things are going to be a certain way do, not as they say. That means it is unlikely for e.g. Martha's Vineyard to end up fully underwater when global catastrophes/climate shit goes down given the fact that the Obamas moved there to a property which lies about 4 metres (13 ft.) above the high tide line. With an average sea level rise of 3 mm/year it will take a while for the island to be flooded.