They should be suspended over deep water, 60' or deeper. You can jig for them with tiny jigs like the ones used for ice fishing, or drop a weighted line with a #6 hook and a live minnow.
There's a thing about targeting Whitefish in the summer, only take the ones you intend to keep out of the water. If you don't want the fish, unhook it without taking it out of the water or it will die... these are very fragile fish when it comes to warm weather.
Good luck and let us know how you do!