Drowning is a horrible way to go. If you decide to drown it, do not walk away. You must stay and watch as it frantically tries to get out of the cage and struggles, then its last breaths come up as bubbles and it shudders and spasms. You owe it this respect, even if it is a predator. Why did you trap it with no plan to deal with it? Please have someone shoot it. Good luck.