It isn't that the White's Surfmaster Pro isn't good for fresh water, it is just that there is more iron trash in fresh water that takes longer to rust away. You can wear yourself out digging tiny pieces of iron and bird shot.
One of the best for fresh water is the Excalibur because it is almost as deep as the Pulse units, but has superior discrimination. Also very good for fresh water is the CZ-20 & 21, 1280. Also the Tiger Shark and BHID. I didn't like the White's BHID only because the coil floated to much while the coils from the others don't have to be forced down while in the water.
On the coasts where there is lots of sand movement a Pulse handles the salt and black sand well while reaching down for deep targets like wires, screws and anything metal. You really need different tools to enjoy the hobby the most.