How can I connect a N6-30 plug from a L14-30 receptacle?
Rather straightforward, since all the power pins your water heater requires are present, and ampacity is the same. 

If you can't find it, hit the local electrical supply house and get 


the appropriate L14-30 plug 
N6-30 socket for cordage 
10/3 cordage of your desired length, do not use Romex


A big-box store is not an electrical supply house.  

Assemble per instructions and done.

Nothing goes on the 14-30 neutral pin.