I am using unstructured mesh of 2.1 Million grid with sea level conditions.

After running 50 iterations Fluent displays the messages like

Abslute Pressure limited to 1.00e00 Temperature limited to 100K Time step reduced in 300 cells

And continued till end (alhough not converged)

What should I do?

This typically means the solution is going numerically unstable somwhere (may be only for a few of mesh elements)and the solver is struggeling. If the solution is just starting, it may be that the flow field tried to permutate too drastically. You can try to recover by: 1) reducing under-relaxation factors to low values to start a solution... or 2) setting variable limits to more reasonable ranges... 3) "Patching" the pressure and/or temperature region to a more reasonable value.... or 4) examining your mesh for ill-posed mesh (run mesh quality checks in your meshing package)...repair poor mesh as appropriate.