Greatwhite,
Not sure what your priorities are in narrowing down your search. If price is one of the major factors, then the Sandshark would be a good bet. Being you want to use it on the beach, I would imagine that a waterproof unit is also a top priority.
If you want to dig all targets and go deep, then a PI is a very good option, but there are other VLF machines that discriminate that are very good.
If price, waterproofness and PI are the top 3, then the Sandshark, along with the Detector Pro PI ( that is what I have as a PI unit) and the Whites PI pro are close in price range and all work very well. I use the Detector Pro for the main reason that my shoulder is shot and I love the light weight. It also has an adjustable frequency so you can tune it to the conditions. It is very stable, very deep, tuned to hit gold and I believe Eric Foster helped with the design. I do not dive but hunt in knee to chest deep water when applicable, but I am not sure how deep it is rated for. I do think some people had trouble with the headphones leaking, but since I don't go under, I did not care. The weight, ease of use and freq adjustment was more of a selling point for me. I hunt the beaches of NJ and I have NO problem with the PI except when I am in the wet sand and there is lots of iron. Then, it's either get out the Excal or bite the bullet and dig tons of holes. I do know that a PI is indispensable when the beaches are sanded in....they just do deeper and end up getting targets when the Excal can't. Also, I have to hip mount the Excal because that baby is heavy.
If diving is a concern, then I would lean towards the Shark or Whites. The Sandshark does have the lifetime warranty...a BIG plus if you hunt salt water because that stuff can wreak havoc on a detector that continues to get submerged.
Just one thing....be prepared to dig LOTS of junk, iron, etc. that is DEEP if you hunt in the wet sand or ankle/keep deep water. If you go out further, there may be less trash. But in the trash, it gets to be a bear since the PI's hit on everything. Not sure how the limited discrimination of the Infinium works, but that unit seems to be out of your price range ( it was for me too !).
I hope this helps. To me, if you have access to the beach, want to get out in the water where most others do not go, are prepared to dig everything.. a waterproof PI is a must. They do find the gold.
One last thing... you could get a used one, but I am not sure how you feel about that. Me, I am a little leary about purchasing a use water machine.
JC