So I have a ESP LTD M-50 and it has a black satin finish. Which shows up finger marks really bad.

Should I just be using water on it?

Also, it shows up little pick marks where the pick has hit the body, and they don't seem to be coming off.

Any help would be great

Pick up a guitar polish at a music shop or online and use a lint-free cloth- that will get rid of all the finger marks. You can use it on the pickguard to, but the pickguard is there for those scratches. That's just going to happen- there's nothing you can do about it unless you want to be that anal and replace your pickguard all the time
Dunlop makes a total care kit, comes with body polish and fret board cleaner and a few cloths - works awesome

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Tascam US144

Just do what the guy above me said,

you can't get rid off pick marks, but you can prevent them.

Just don't put so much swing in your picking hand unless you've got good control, your pick shouldn't be hitting your body when you play.