I don’t think you’re correct on the water requirement.
I allowed a gallon per day. Your 1/2 gallon might be ok for water only, but not IMO if all food is dehydrated, as it obviously is if he’s surviving on a little more than a pound per day.
I’ve had some experience in food and water requirements due to experience in desert backpacking. I used to carry a minimum of 2 to 3 quarts per day, but that was generally with undehydrated food.
In hot or dry weather much more.
Here’s a water requirements calculator.
http://www.csgnetwork.com/humanh2owater.html
I’m not familiar with water purification equipment of the type you mention. There is a feedback factor here. The more power needed to purify the seawater, the more he’ll need to work to produce the power, which means the more water he’ll need.
I’ve read Kon-Tiki, and from their experience it looks like, at least in tropical waters, he won’t have any problem with getting enough food to survive.
There are quite a few ways to process seawater with small equipment by osmosis, distillation and condensation. A search of the topic can yield some interesting reading.
A gallon a day would be great, all I’m saying is that he can get along on less, and he might have to.
Yes, operating a manual RO unit requires energy throughput. If it didn’t we’d have free-running RO plants all over the world making the deserts bloom. And yes, that will require work and produce perspiration, increasing water need. The equation must favor producing more water than is used or it wouldn’t make the slightest sense. Your points are valid.
I’d agree that he ought to design and install a system that can produce 128 fl oz per day and hope to get a good fraction of that in practice. This journey won’t be either easy or comfortable, to say nothing of the risk of the unforeseen. I wish him luck.