Question: I want to ask that people who are not buried after death like if they died at sea or burnt but not buried, how they are answerable to munkar nakir and how they would go through their phases of grave?

Answer: 
I had the same question and many still have. You can read Surah Abasa(no.80) Quran 80:21 which says "Then He causes his death and provides a grave for him". Now we need a mind to ponder here it could be/not be what we think like a physical place on earth. Also not only grave, there are people eaten by animals and some other reasons can also be there for death. Also in Surah Saffat it is there "He would have tarried in its belly till the day when they are raised"; (37:144).
Now all it needs a clear mind to think and reflect. Also please keep in mind that always answer cannot be exactly like what we expect or think. Allah knows best what does it mean. We can try our best to understand.
Hi there is no grave punishment.. only soul punishment..thanks
Times stops when you die..after death everthing happens in another dimension..Allah will put you straight away in to heaven or hell not in grave