No, You can put in the pump without taking off the cams and backplate, but it is kind of a pain. IF your engine is just like mine. The backplate in metal, the water pump has a lip on the top edge that sits behind the backplate, about 1/4 inch. I have done it once when I could not get the cam bolts off. The water pump fit on with 3 nuts on the left side, a couple long bolts, there are also 2 threaded pegs, to get the water pump to slide off those pegs you have to pull it straight out, but you can not pull the pump straight out because the top of it fits under the lip of the backplate.
You will have to remove the nuts and bolts, then remove the 2 pegs, you can unscrew them with a small E-Torx femal star socket. Pull those two long pegs out. Now you can work the pump off with a little wriggling. Putting it back on is a bit harder, it does not just slip back on under as easy as it came off. I took out some of the bolts that hold the backplate on, this allowed me to be able to pull the bottom of the plate out just enough to force the lip of the water pump back under it, a little tricky but very doable, just be careful when forcing it in that you are not putting any pressure on the new metal gasket so as not to damage it. You may need to use a screwdriver or something like a needle nose plier with an angle at the end or something else that you can use to get behind the bottom lip of the back cover to pull it out to create space to fit the pump in.
So yes you can do it, but you will have to take out a few bolts from the backplate, Now this brings up another issue, the bolts are behind the cam pulleys. The pulleys have spokes with open spaces so no problem accessing the bolts, but if it is lined up like mine was where both bolts just happened to be lined up behind 2 of the spokes, then you have to turn the cam pulley maybe up to 10 or more degrees one way or the other to get your socket on it. Turning the cam even a little you run the risk of forcing a valve to hit a piston possibly and doing damage, The manual on my engine tells me to turn my crank shaft counterclockwise 60 degrees before messing with the cam pulleys such as trying to get the cam bolts out so if they move you do not damage anything. I would for safety go ahead and do the same, you will have to find the repair manual instructions for your engine, On mine, 60 degrees was exactly 2 teeth on the crank shaft. Then you can mess with the cam pulley. Just remember to get everything lined back up when you put your belt back on.
one more thing, with the back plate on, I could not fit the metal gasket back in place without risking damaging it due to one of the little pegs on the gasket, I had to cut the little peg off with a wire cutter then it fit on flat without any forcing.