Also - a lot depends on your normal daily intake of water.
If you are already a fairly well hydrated person, the water from an enema (especially if you add a little sea salt) will not cause a problem.
If you are a person who does not normally take in much liquid during a normal day, then the water in a plain tap water enema may indeed cause you some problems and you would likely notice increased urinary output as one side-effect from this.
I am a big water drinker - normally drinking 4 to 6 twelve oz. glasses per day plus whatever I get in my morning coffee, etc. So for me, a salt water enema or two (usually not totalling more than 2 quarts) doesn't seem to give any ill effects nor have I noticed increased urinary output from this.
Always be mindful of your own body - if you feel 'funny' (lightheaded or headachy) STOP... Don't change your routine suddenly - this includes your water intake. Doctors used to be so quick to insist on 8 glasses of water a day (it's actually just 64 oz. of water a day as this recommendation was for an 8 oz. glass), but they no longer do this as that was debunked.
But if a doctor recommends that you take in more liquids for one reason or another, and you suddenly increase your intake by a large amount, then you can have problems. And that would include intake by enema.
So use common sense...and ask if you aren't sure (and providing details regarding amounts is always good - helps us figure things out, too).