Most problems around the pond can be solved pretty quickly with a decent .22 rifle with a scope on it. This takes care of raccoons pretty nicely. If you have bigger problems, like beavers, you may want to move up to a .223 or a .243 or something in that neighborhood.
Muskrats are almost impossible to shoot. They are best taken out with traps. Conibear traps are ideal for this. Drowning traps work. Snares can be good for them too if you know how to tie a good one.
If you want to cover all your bases I would just get a good .223 rifle and a bunch of conibears. Those two items will solve all of your pond problems...unless your problems include bears LOL.
