If there is water around arena - sure.
Since, in your story u stated that you manipulate with water, you can't make water from nothing.
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Also, I just picked up two random fighters and arena!

I'm making a prologue for comic right now (to explain how all of this started), so I won't tell you yet who are those two who will fight first.

To keep you in suspense. >:}
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When I post prologue I will also tell you who are those first two.

A little hint: grey VS wicked!