Dan,
Piranha is a PI that pre-dates the Sand Shark and uses the same coils as the Sand Shark. I haven't used a Piranha, but the Sand Shark is a good performer. It will work fine in fresh water. The issue in some fresh water areas is that iron and small foil trash do not break down as fast as they do in salt water. If a lot of trash has been dropped in fresh water it probably hasn't broken down and you will dig a lot of that trash with a PI detector.
PI's do not discriminate. Even in the salt water areas where iron deteriorates fairly quickly, you will dig some iron trash with a PI.
The Wader or Pirate pro offer discrimination. With either of those set up right you will not dig much iron at all.
I like both a PI detector and a VLF detector with discrimination available that can go into the water. Depending on how much iron trash there is determines which detector will get the hunting time on that day.
In my opinion, $250 for a good working water proof PI in good shape is not a bad deal.
Cheers,
tvr