If you are in Moro Jable you must definitely stop at the Turtle Nursery! Here they take care of sea turtles that have been washed up on the beach. You can watch these imposing animals from close by and have a look in the nursery. Nice tip, if you encounter a stranded tortoise you must call the emergency number 112 immediately, this shows the importance of these animals for the island.
On the way back, take a closer look in the harbor itself. Keep staring at the bottom of the port dock and have some patience... Soon you will see some giant stingray and many tropical fish passing by.