I'd bet your trailer is probably a bit too deep. Try leaving it slightly shallower next time, as well as following dyeguy's advice on moving a little swifter, and see what you've got at that point. Assuming you have a bunk trailer, and it is set up properly, you should slide up the bunks a bit, and settle on them, where you are no longer floating. Then, walking around in the boat shouldn't be an issue - it is cradled and stable.
There is always an angle issue when loading boats, as the boat is floating on level water, but the trailer is on an incline. This often has boats floating in under the winch post, or sliding around a bit. As a result, folks often try putting the trailer deeper, when in reality, shallower will help the issue.
As far as powerloading, my interpretation is that the original poster just wants to leave the boat idling in gear, and not actually powerload in the sense of goosing the engine to slide up the trailer. I can't really answer to the legality of idling in gear, but I don't think I'd want to leave a motor in gear, without someone at the helm, as there will be the one time, somewhere down the road, where something will go wrong.