Before anything, I suggest if beach hunting, getting a waterproof unit. Not just for the peace of mind that if you drop it in the water it will be ok, but also for the fact that you can thoroughly rinse it off with fresh water after a hunt in the salt water. And you can rinse off ALL components ( meaning head phones too). Salt water and sand wreak havoc on detectors and it's best to be able to clean them off really well and let them dry. Having used my DFX on the beach ( it is a really good beach unit when set up correctly) and my friend who loves his Sov GT on the beach, they are subject to the salt air and sea spray and you cannot rinse them off. They end up getting corrosion on the head phone jacks, possibly start to get some inside the unit, etc. Your call, but waterproof machines, though more expensive, are just better for beach hunting.
Of all the machines mentioned, you have to realize there are big differences in PI units vs VLF units. If you are going to strictly hunt in the water, then the PI is the way to go. Much less trash. If you decide you want to venture into the dry sand, then the PI will start to wear on you due to hitting on every target. Yes, you can start to somewhat be able to tell differences in the targets like nails, iron, etc. but the more trash there is, the more there will be nails and iron targets on angles or sitting in a position where they sound good.
In all honesty, there are pros and cons to all detectors and only you can prioritize what features you like the best. If depth is the #1 feature you want, get a PI with a big coil like the Dual Field, Infinium or HH PI. If you want discrimination with tone ID, then the Excal, BHID300 and CZ21 is the way to go. If you want a more easier to use, light weight machine with discrimination, then the Detector Pros Wader with the 10 inch coil is a good buy.
Saying all that, I would suggest if this is your first go around hunting beaches, to stick with a good, waterproof VLF unit. You may get discouraged very quickly if you first use a PI.
Although you will not get the depth of a PI, you can always use an Excal, BHID300 or CZ21 in all metal mode to see how a PI may 'feel' ( and the BHID has a form of discrimination in all metal in that the lights on the machine still work although it is single tone. I have a BHID300 and when all metal hunting, when the red light flashes, the target is almost always iron or junk...it's a pretty cool feature). Still not the same as a true PI, but hunting in all metal does tune your ears a bit and you will realize how much more garbage you will dig up when using a PI.
JC