If you are serious about shipwrecks and want a pulse detector, then THE choice for you in your price range is the Garrett Sea Hunter. It is far and away a better detector than the Sand Shark. It is not even close.
The Sea Hunter has interchangeable coils, removeable headphones, a battery compartment completely separate form the electronics and, perhaps most importantly, the coil can be moved VERY slowly in standard mode. It's depth is at least as good as the Sand Shark's.
The Sea Hunter also has an adjustable pulse delay that can be used to knock out shallow surface trash.
Mel Fisher's group still uses the Garrett Sea Hunter.
The Sand Shark has a fixed pulse delay, hard-wired headphones, a hard wired coil and changing batteries in the field can be a real nightmare as the entire unit has to be opened up. It also detunes very quickly over the target which is a big draw back in low visibility conditions. It is an OK, beach machine, I suppose, but really it's only selling point is the lifetime warranty.