Q: How important is shade?
A: Absolutely critical. You get significantly warmer in direct sunlight and sweat more. The pictures of the boat show a fiberglass box in the middle which he could have sheltered in, and any type of canvas would have helped keep him out of the sun.
Q: Alvarenga described catching turtles, fish and birds with his hands and eating them. Is that plausible?
A: Over time, the underside of the boat would have become its own ecosystem as barnacles, seaweed and jellyfish collected there, which in turn attracts other creatures. How often can you grab a turtle or catch a fish with your bare hands? I don't know. Bird blood is no more salty than human blood, so would have provided some hydration.
Q: Without fruit and vegetables, wouldn't he have developed scurvy?
A: Actually, unlike humans, birds and turtles make their own vitamin C, so fresh meat from those creatures, especially the livers, would provide sufficient vitamin C to prevent scurvy. British sailors used to get scurvy because they ate preserved meat which had oxidized and lost its vitamin C.
Q: Wouldn't he get skin sores from all that water?
A: He'd need to keep mopping himself off and stay dry to avoid that. People on life rafts, or say a piece of floating wood, can develop real problems with macerated skin. Staying out of the water is a huge advantage.
Q: There's some suggestion that Alvarenga was a large man before he left. Would being overweight provide an advantage?
A: It would be a significant advantage. He could live off his own body fat and muscle for a long time, so long as he was able to get some water, vitamins, micronutrients and a little protein.
Q: Didn't he look too healthy, even a little bloated, when he arrived?
A: The appearances of malnutrition can manifest differently depending on how short you are on calories or protein. Some underfed children in Africa look like stick figures, others get swollen. It's only in end stage starvation that people get that really emaciated appearance.
Q: Alvarenga seemed to give confused and contradictory answers to authorities. What kind of psychological effects would such a journey have?
A: I'm not an expert in psychiatry, but we all have the feature of resilience. It can be trained or even learned on the fly. For instance, soldiers learn to deal with combat horrors. Presumably he was out on the ocean every day as a fisherman before he went missing, so he would have been familiar with the environment and with adapting his behavior to the elements.
If he had nutritional deficiencies, he may have developed some dementia or other syndromes which compromised his mental state. I'm not surprised that some of the answers he gave were a bit off and he wasn't able to remember things.