Nick, I also ride a DRZ and so far I have been lucky enough never to drown it but before I owned mine I saw someone else drown one and I would say this. All the advice given above is sound but don't rely on just carrying any old plug spanner/socket with you for the DRZ.
The spark plug is incredibly difficult to get at unless you have a suitable tool so try this in your shed/garage/garden or whatever before you have to do it for real in the field so to speak!
As long as you can get the plug out though you should be OK. I've seen quite a few drowned bikes over the years (mostly on Strata Florida - which wont be a suprise to anyone who's done it) and only one was damaged and that was because the rider didn't get all of the water out before putting the plug back in. The engine actually fired up but it almost instantly blew the crank case seals (it was a two stroke obviously).
If the worst does happen and you feel yourself going down into the water SHUT THE THROTTLE!!! As long as the motors only ticking over your unlikely to suffer any mechanical damage.
Also if the water is that deep, consider getting off and walking the bike through. This will unload the suspension and raise the air box several inches. It also means that if it starts getting really deep you have the chance to turn around before you disapear
Of course the down side is that you do get very wet but then you do get very wet when you fall off in a river as well don't you.
Either way don't let it put you off. This kind of thing is all part of th efun of trail riding.
Cheers
Richard