Comment by coconut_crab

  - settlements on river banks will be swept away
  - the biggest nuclear power plant in Europe, ZNPP, won't be restarted due to lack of cooling
  - Crimea will lost access to fresh water
  - First line of defence for both sides are swept away.
  - it will be harder for Ukraine side to mount an amphibious assault

So it's a lose-lose situation for everyone.