That Nautilus Lifeline kicks butt ...
Nautilus Lifeline Diver Radio and GPS
... along with the other very useful features, the waterproof to 425ft closed, 3ft when open, is outstanding ..
no need for a bulky cannister
(note: this is line of sight, radio communications. It does not link to a satellite on the 406MHz band and as such it might not be suited to "diving at the end of the world" dive sites) .. $300
The McMurdo FastFind is a 406MHz Satellite GPS Personal Locater Beacon (PLB) ...
Quality Diving Equipment by Custom Divers - Fast Find 210 PLB and while a bit more expensive, it can be used at those "end of world" locations .. $220 at Landfall Navigation ...
FastFind 210 PLB w/ GPS ... only waterproof to 33 feet for a short time, it's going to need a cannister