If you are using water from a hose, then it’s pure and no need to worry about anything becoming impure. But if you are using water from a water can, then yes the water which comes into contact with the najes area becomes najes and you must try to avoid having those drops fall onto another area of your body. As for the water splashing, if you don’t have certainty that those drops which splashed are najes, you may consider them pure.