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.
As far as punishment for soul those who body is burnt seems to have some validity but what about questioning in grave and all the fear that mullah puts in your heart about grave. that does not seem to apply to those whose body is never buried in grave.
Please only reply with sahi hadith with reference or from quran with reference do not just say what your little head think and understand