I can't comment on the depth comparison. Perhaps the shark is deeper than the 600 ? The shark is a PI after all. And it's always hard to argue with pulse's depth and versatility on the beach.
HOWEVER : The immediate benefit on moving from the shark to the 600, will be the ability to pass iron (nails, etc...). For that reason alone I would always prefer a discriminating machine. At least on the beaches I hunt, where a person might go psycho digging all the bobby pins, bent nails, etc....
And the 600 also retains an ability (if you choose certain option/settings) to find some incredibly small stuff (ie.: micro-jewelry).
If micro-jewelry isn't your objective (so that you're not bedevilled with teennnssyy foil turds), then machines like the CTX, Excal, Sovereign, etc... are also killer beach machines in the wet-salt.