Saralou, determining how much you can do in one day depends on several variables:
1. How long your ship will be at that port.
2. Whether you dock or tender at the port.
3. The duration of all the shore excursions.
4. When you get off the ship that morning.
5. Transportation between the excursions.
Docking gives you more time to explore than tendering. You must be back on the ship, according to veteran cruiserss, 30 minutes before departure. So if you want to do some long shore excursions such as a HOHO bus tour you should save them for dock ports. But I see no reason you can't snorkel on your own and control how long you do it.