They can be pain agreed. WD40 is no use, try PlusGas or similar. The steering shaft is slightly tapered and there's a woodruff key on there too. If the boat is new to you, goodness knows the maintenance history? Hopefully whoever fitted it, put a smear of grease on it first.
Undo the central nut and washer. Now put the nut back on so it sits flush with the steering shaft surface. Couple of reasons, it protects the thread on the shaft when you hit it, and if this comes off you'll go flying otherwise. If you have someone to help, get them to haul the steering wheel with hands at a quarter to three angle. Hit the central steering shaft with a couple of glancing blows with a hammer. It should come off.
If that fails, there's always Stu's approach!