There are actually several ways. First, most steering wheels have three positions that they can be locked down with using the center nut. This of course assumes that one of those positions works for you. First, straighten the drive. Next, loosen - but do not remove the center nut. Then, using a rubber mallert - tap the steering wheel from the back side to loosen it. When it's loose, remove the nut, the wheel, then clock it before tightening it back down. (You leave the nut on loose to avoid stopping the steering wheel with your face.)

A better option - assuming you have a steering rack that drives the cable - involves straightening the drive first again. Next, loosen or remove the four 7/16" nuts that hold the rack up against the cable. Carefully turn the wheel to the straight position, then bolt the rack back up. It takes about 5 minutes if you have decent access to the rack.

I know hydraulic can't be centered, but why?

Because unlike cable steering, which has a mechanical linkage between the wheel and the drive, hydraulic steering uses fluid movement through lines to move the drive side to side. You get some bypass in the helm and thus, no center. I have Latham hydraulic steering on my boat and not having a "center" hasn't bothered me yet - and I'm pretty much OCD.