I'm pretty sure the afternoon dive is an add-on cost. The morning dive is a 2-tank dive. There are places that have shore dives, but you have to pay for a tank. I think at Cozumel Palace, for instance, it's $10 for a tank. At ScubaClub there are no additional fees for shore diving (maybe extra if you wanted nitrox?). And yes, there are people who will pay for the extra afternoon dive (and traditionally, it costs extra at other places too) but maybe not for every day they're there. If you go to Anthony's Key in Roatan, Honduras, they include 3 boat dives per day in the package. Never figured out the shore diving there and don't remember if it was included (I think it was?)..