There can be some good diver shooting on the prairies, but most people don't bother with them since there are so many mallards. If you really like diver shooting, bring your boat and the big decoy rigs. The taboo about shooting the roost does not apply to divers, just mallards and geese.
Mallards can be hunted on small ponds as well, because of a phenomenon that happens on the prairies - alkali sloughs. The water in the big roosting marshes is often quite salty and alkaline, and is not good for ducks to drink.
Most of my mallard shooting is on small watering sloughs. Birds will go to water just at dawn, and again after feeding in the fields but before returning to the roost. A dozen mallard floaters are all you need, and 6 is probably enough. Again like field shooting, it is critical to set up exactly where the birds want to be. A boat is not needed for this type of duck shoot, only some chest waders.