I'm a far far cry from any kind of a crappie expert. Much more of a hybrid and striper fisherman. But I have spent a ton of time on Hartwell. Generally navigation is very safe on Hartwell. There are red and green channel marker buoys in the main river channels and many of the creeks. Once the creeks narrow it goes to a single center channel black and white buoy. The lake level can vary by 20 plus feet so during drawdowns, you will start to have quite a few unmarked shoals begin to show up. Hartwell is also a flooded forest. They left a good bit of the timber standing in the deeper water and when the lake flooded to 20 or so feet of the surface they topped the trees. In most areas shallower than 35 feet they clear cut the timber. So you generally speaking have cleaner bottoms out to about 35 feet then the timber will start as you move out deeper. There are areas where they missed topping the trees at 20 plus feet and when you get a big lake drawdown beyond 18 feet or so you can have massive tree stumps right out in the middle of the channel in very deep water. The lake gets dangerous at this point. Going up in some of the creeks there is a little timber left standing in the shallower water. Specifically, creeks like Martins Creek, Gum Log, 26 mile, and even a few coves in Conneross have isolated standing timber. At the mouth of Saddlers Creek there is some standing timber just under thesurface you have to watch out for as well if you try to cut between some of the islands. Be very cautious if you decide to run way up the Keowee River. There are a ton of stumps up river from the Bayshore community. In Conneross there is a very long underwater point that comes all the way to the center of the creek with a house foundation on it. The Channel Markers are off to the side of it but a ton of boats miss the fact that the channel marks have shifted so far to the side and cut right across it. Once the lake is 10 feet down, that spot is very dangerous. Seen multiple boats hit it running wide open.
To aid your navigation I would highly recommend a Navionics card for your GPS. You can then clearly see all shallow areas you should avoid. It will not show the places with standing timber during draw downs though.
As far as the little bit of crappie fishing I have done on the lake, there are tons of bridges in the creeks that the pilings will hold fish for good portions of the year. Then in the heart of winter deeper water docks begin holding big numbers of fish under them. The creeks that typically get stained after big rains tend to be a good place to start looking because they hold the most bait. In those stained creeks you will see a good number of boats pulling jigs in the early spring, but I have never fished that way so i can't comment on it. Many many points on the lake have cane piles sunk on them by bass fishermen out in about 20 feet of water that will also hold crappie.
Good luck.