How do I couple IMC underground?
Do I need to use any sealant on these threads to prevent water
  ingress?


No you should not use any sealants, chemicals or compounds that are not specifically approved for electrical installations.


  How do I couple unthreaded IMC underground? Do I need to use liquid-tight fittings?


The best way to couple unthreaded IMC is with threadless IMC compression couplings. Yes they do make them and they are expensive and they are not a stock item. Exercise caution when using them. They have a tendency to pull apart during expansion and contraction of the soil. Try and use them where there is less of a chance to do this, meaning at the beginning and end of the trench around the 90 degree ell coming out of the ground or use with an expansion coupling.

Good luck.