A bunker like an oil rig is good, as long as the anchor link is adjustable. After the math, we sure don't want to end up a couple of hundreds feet below the sea level, do we?
I think a mobile bunkers (unsinkable boat, submarine, special design van, balllon???, etc.) is better than static ones. we really can't depend on what the end result to the area where our static bunker located.
One may survive on a sub at first....but where do they find safe harbour to replenish...
If a big disaster indeed happened, I think all the survivors (land, air, sea) have to start all over again.
For survivors in sea area, fishing probably a good thing to do.
For land survivors (assume there is still productive land left), they can start to plan and grow something.
From what I read in this forum, if Yellowstone erupt, half of U.S. probably will become unproductive land for the next 10 years.
In my country, recently Mount Merapi erupted, and the land in min 10 Km radius probably useless for a while. Luckily it's not a global disaster, so other people still can help the victims to start their new life.