When you jump into ponds, lakes or seas (deeper waters), you receive a message “you’ve been knocked out, a day’s rest should get you back up your feet”. A day passes, and you begin outside your house again. This loop also happens when your health points have been depleted. This only happens when you encounter ‘dangerous’ slimes. These slimes can bump to you, or curl up in a ball of spikes. If you don’t have the right tools for these encounters, the slimes can hurt you.
As for the slimes themselves, you can’t really harm them. When you throw them in the water, they disappear. When you shoot them in the air, they just bounce when they hit the floor. You can, however, dissolve some slimes with water (or other substances). When this is done, they just disappear in a few drops, leaving behind some crystals or slime essence.