Having hunted fresh water lakes here in Texas, I do know that, had I a PI machine, I'd go crazy.
Work the beaches to get a feel for your machine. Learn the differnet sounds it makes on targets. Then venture to the fresh water and park.
If you started in the park, I think you would dig so much iron and junk that you might give up detecting out of frustration before you had a chance to hit the beach.
When I get to the beach I hunt with my BHID in all metal. In the lakes I hunt in Disc. and switch back and forth between the two when pinpointing.
Coolwhips