I haven't moored a boat in a lake, but I moor a 21' fiberglass boat in the ocean every summer. I use a 75 lb. mushroom anchor with 10 feet of 1" diameter chain, followed by 25 feet of 1/2" chain then a swivel then a mooring ballon with a 1" diameter rope that is 15 feet long. Once I drop the mushroom anchor in the water each year, I attach the mooring rope to the bow and let the outboard pull hard in reverse to drag the mushroom enough to fill it with mud, then it always holds perfectly and my boat has weathered some really rough storms. You'll want either a self-bailing floor or a bilge pump if the boat is moored during rainy weather.
Good luck,
Salty