Well a two acre pond liner would make a dive trip look cheap, so would enough barley balls http://www.gardeners.com/Barley-Ball/PondMaintenance_PondCleaners,32-576,default,cp.html to keep it clean.
Before pond liners the English used a heavy putty like clay to build ponds. Avoid anything loose that would be stirred up by the first diver in. Some bottom feeders to keep things clean and surface feeding fish to catch insects and larva. I put 10c feeder goldfish in my bathtub the horses drank from, amazing how big they got eating mosquitoes! Any run off going into the pond should go into a catch basin first to let the silt settle out.
A dock or paved entry will keep things cleaner on shore and clearer in the water.
Provide lot’s of parking and picnic tables for all of your new dive friends :)