Sandsifter,
There is lots of info about these 2 detectors on the forum. The bottom line is, the Infinium is a PI detector and has very little, if any, discrimination. It will go deep, but will find small bits of iron, steel, etc.
The Excal is a disriminating detector, and can be run with very little discrimination which will have the detector null out on iron only, or you can turn the discriminating up. It is a multi tone detector and after using it awhile, you can distinguish between the lower conductive targets ( lower tones ) to the higher conductive targets ( higher pitched tones).
Both are water proof, but the Infinium may go a bit deeper and is more sensitive to gold.
If you are in an area with a lot of iron trash, steel screws, etc., your better off with the Excal.
If you will be hunting in the water at least 80 percent of the time, where there is less trash, the Infinium may be the way to go.
JC