Glenn, the Ace 250 was a great choice and is one of the four machines I currently own, although mine is a generation earlier model that is not quite as fancy. For the price they are a good value and are easy to adjust and learn. One thing you are going to find is that no discrimination is perfect and there is much more to be gained in knowing your machine than in buying a machine with more bells and whistles. What I would recommend is setting up a little "test garden" somewhere in your lawn. You are going to bury some "known targets". Suggestions: small steel nail, large steel nail, 5" length of barb wire, horse shoe, soda can, bottle cap, pull tab, aluminum foil square, spent 12 gauge shotgun shell, penny, nickel, quarter, large civil war style bullet, silver quarter, piece of gold jewelry (don't worry, you'll find it). Bury them all the same depth, about 6-8 inches and a couple of feet minimum apart. You can mark the locations with golf tees if you like. Now "learn" what those targets tell you. Play with the discrimination and see what it does. See how the signal reacts to the target. Don't worry about focusing on the LED display, just listen as you sweep back and fourth and up and down. This experience will more than pay for itself, the treasure can wait. After you are comfortable dig the targets up and have someone else switch the holes they go in. Try to then use your experience to identify the targets. If you are patient enough experiment with depth too.
You should be able to find some coins at Morgan Falls. It is good practice to dig most of the targets you get when you are learning, but it can be frustrating. You can turn the discrimination up high enough to avoid most trash, but all you will find is pennies and clad coins. You will miss nickels and unfortunately, most of the gold jewelry because it is in the foil and aluminum range. Hey, if it was super easy everyone would have already gotten everything. Be patient. Also, practice good will by filling in your holes well. Don't tear up the grass plug any more than you have to. I guess I'm preaching to the quire since you are in landscaping; however when you are learning (or when you are lazy) you tend to dig a bigger hole than necessary.
I have looked around the field you speak of on Johnsons Ferry if you are talking about across from Columns Drive. When I was younger that grass field was regularly filled with cars on weekends when rafting the river was in its heyday. You can find coins there. Look around the trees where the concrete ramp goes down in the river to. People at one time used to line that bank just hanging out.
I live near Wheeler High School. What kind of stuff are you most interested in finding. New coins, old coins, jewelry, civil war, old artifacts?
I am also a diver and do some underwater metal detecting occasionally. There is a place called Huntcliff just above the dam from where you were talking about that I might dive sometime. We used to jump off the cliffs up there. Lowest cliff was 45 feet and there was another at 60 feet. I have to think that a lot of junk is under that cliff. It may be too dark and murky though at depth, not to mention the dangers of giant catfish!
Stan