I am not sure if you are new to detecting, or have some or lots of experience, with both land and water. I am going to play it safe and assume you are new to detecting with no experience. If I am wrong, sincerely sorry.
Pi's are great machines. They really perform well where single frequency machines don't. Some of these locations are wet salted sand, salt water and high mineralized soil, very typical of areas where you can do some gold nugget prospecting. PI machines also sound off on
all metallic targets. And because PI's tend to get better depth in these tough to work areas, they also sound off on many many signals. You can
not notch out or discriminate the iron like you can a single frequency. So,.....why do some people do so well with their PI's? Simple......they use them in the right location. I don't mean where the most rings, coins and jewellry are necessarily lost, but in areas where the junk is most minimal. That would be (but not limited to) the deeper water or beaches that have been hit hard by other users of metal detectors. You will be looking for good targets out of the (depth) range for other detectorists. If you constantly used a PI on a very busy beach or other busy area, the hundreds upon thousands of signals would either burn you out. or your good to bad ration of finds would be so terrible that you might as well go eye-balling for targets.
Now enter the INFINIUM.......It is also a PI, but it must be ground balanced (not the Mark II) and a stable frequency must be chosen (not on the MARK II). It also has a feature, where it gives a double beep on targets. As a rule....high-low signals read as gold and rusty metal. Low-high signals read as copper, silver and iron. So, iron tends to read on both types of metals. The Infinium also has an iron check to also help identifying iron. But the learning curve for an Infinium is long, and I have seen many people sell their machine, saying it isn't working, only to be snatched up by someone else and really do well.
So...my suggestion is stay away from the Infinium if you are new. Get a Mark II, use it in the proper environment, buy a good expensive water scoop, dig lots of holes and build up a good physical endurance to performing these tasks. If you do this, and still have the fire in your belly to water detect, get the Infinium. Many have taken this route and moved on to a better PI, and many back-tracked a bit and realized that water hunting isn't for them. There are no losers, just a bunch of people having a lot of fun metal detecting.
A quick slide show below on my Infinium hunt from last year (give it time to unload)
http://www.photoshow.com/watch/ZT8SI9HY
