Breaker Breaker, Which should I use?
You're in a QO panel; fortunately has unique busing so you can't misfit a breaker that doesn't belong, you can use either the OEM Square D QO230, or one of the UL-classified competitor models the CHQ230 or ITED230.

Since both wires will be hot, you must use tape or paint to re-mark the white wire a color. Typically this is done by wrapping it with black tape.  

Ground first, not last

You should hook up the ground wire first.  You said you hooked up the hots, then hooked up ground, then had a problem.  You were lucky.  You could have been shocked if you tested it without grounds present.  The fact that you did grounds last makes it seem like the ground is at fault. It's not.  It's simply the "canary in the coal mine" warning you of the problem.  Hook it up first, and the problem will be more apparent.  

Divide and conquer

Unhook all the wires, except for - anyone, anyone, Bueller? Ground wires.  That's right, leave all ground wires attached.  

Now, hook up one wire to the disconnect, open the disconnect, and see if the circuit can power up.  If so, power down and attach the other wire to the disconnect, and test again. Now close the disconnect and test again.   Now attach one onward wire to the water heater.  Test again.  Etc. etc.  

At some point, adding a wire will cause the breaker to trip when turned on. Divide that further if possible.  Now you're pretty close to the answer.  If you're still stuck, ask.