In 35 years I've used many different types of machines for for all kinds of hunting, you can run a few tests of your own to determine which machine works best for you, PI machines work great in Salt water wet sand but are not sensitive to small gold, Small earrings, chains, bracelets, etc. Single frequency machines do better in wet salt water sand, but when I say "small gold" I'm talking chains or earrings or maybe even a broken ring. I have a Nox 800, but prefer my XP DEUS II for salt water beaches, that being said, my XP DEUS I with Elliptical coil will pick up anything so small it might be considered a fragment..
but on a wet salt beach it makes too much noise to hear those small targets ...
(
and if your hunting beaches a lot, and Never find small gold, then your machine almost certainly is NOT picking it up)
Two months ago I was on Hilton Head beach and hunting (DEUS II) and saw several others walking around, Nobody was finding much of anything but a few coins that I saw... but I dug a small Gold looking earring, and later I stopped another guy who was using a NOX 800, I said, what do you get here, and held my hand out with it closed in my fist.. swung it in front of his coil, and he said... Nothing !! I get nothing !! I said
really 
He said yes, I don't pick it up at all..
(to be fair, I don't know the settings he had his machine at, or if changing his settings would have given him this thing)
I opened my hand and showed him the earring and he said, oh that's gold, and it sure looked like gold... (it was not actually gold when tested at home) He was amazed his machine did not pick it up... And for my part, I was amazed that My XP DEUS II actually did pick it up.....
For those who want to try some experiments (to see how your machine reacts)
Get some small gold pieces.
(make sure its GOLD and not silver w/gold plating) like a gold chain (without anything attached, no medallions or crosses or charms) and lay it out on wet salt water sand (or anywhere) and see what your machine can pick up. Try small gold earrings, try a small bracelet !!
But here is the real test that gets most gold hunters up in arms...
(if you have a scrap gold ring you are willing to cut) take a small gold ring you found,
cut the band and separate the opening so they don't touch... you can clearly see the band and the amount of gold hasn't changed but I doubt that 9 out of 10 people with anything but a true GOLD machine can pick that up...
My Deus I can pick it up, and special gold machines can pick it up. But no other machines I've ever had can pick up a cut gold ring... (Nothing. (accept gold machines) Fisher CZ, Whites Eagles, Whites DFX, Spectrum, Tesoros, Minelabs, Nox, Garrets, NOTHING)
Try this, it makes for very good conversations ha ha ha