Hi José,
Next time just remove the thermostat (don't put in the spare).  It is not too important anyway as it simply allows the engine to come up to temperature more quickly by restricting the cooling water flow - once the engine is hot the thermostat is irrelevant. So, with no thermostat you can clearly observe the flow of coolant by leaving the cap off. Then you can differentiate between a failed cooling water pump and a bad temperature sender (or connections in its circuit). As you noted, you had already eliminated a raw water problem. After you sort that out you can test the thermostats by putting them in boiling water and seeing if they open and maybe not have to order a new one (pretty unlikely you've got two failed ones). 
Going diving in Bonaire?  I'm jealous.
Ciao, Craig  - SN68 Sangaris, Tropic Isle Harbor, FL