Using the Nuclear Bomb twice pushes the team health bars off the screen
From Worms Knowledge Base
In WormsDC, the Nuclear Bomb increases the water level by decreasing the vertical space of the available screen. As the team health bars are locked to the vertical top of the map, using a Nuke once means that the top-half of the health bar display is rendered above the top of the screen and is thus rendered invisible. This doesn't affect two-player games, but in three- or four-player games two of the health bars are pushed out of sight. Using the Nuke twice pushes the health bar display off the top of the screen entirely, making it impossible to see.
Of note is that the WormsDC Nuke can only be used when there's more than a screen height's worth of map to sink (usually five uses). Any attempt to use the Nuke after this point results in the fanfare playing, a brief and quick rumble, and no visible change to the available play area. This is not the case in Worms 2, Worms Armageddon, or Worms World Party, as unlike first-gen games the space below the water surface is still rendered as well as a significant amount of space above and around the terrain itself, meaning that the water level can be raised to fill the entire map.