I probably would not take a taxi on the dockside without doing some research first about exactly where I wanted to go and what I wanted to see. Suggest you check out the ports of call section on cc for some ideas. There also are bound to be the names of some recommended guides.
We were in this area in November and I would advise the ships shore excursions. the taxis were quite a walk from from the dockside, With one group who went alone, they got half way to Panana City, ( which is a good 2 hours way on bad roads) the taxi driver then said he was ill and turned round and head back to the docks and still wanted paying. Also donot forget the ship will not wait for you, if you have gone by taxi, but will on ships excursion. On our ferry and locks cruise we were 4 hours late back and arrived with police escort. Hope this is helpfu.