@Vaqvbu1a
Don't you know that saltwater is denser than freshwater?
That means that when you dig for water near coastal regions you will mostly get saltwater.
His magic power was somewhere below rank D, I doubt an ember can keep up the boiling process of desalination for a liter of water without at least a day.
Also the 2 day travel is from his starting point to the wetland not from the beach to the wetland.
Besides wetland water being toxic is due to the pollutants mostly at the top layer, with nothing he can take chances with the one underground.
Comparing that to boiling saltwater until he runs out of energy should be more agreeable. Also in the topic of boiling, for desalination he would need to boil everything up and only capture the steam(the salt stays at the pan and the freshwater is the steam) however for wetlands you only need it to boil enough to kill the bacteria, debris can be remove using cloth or letting it settle in the bottom.
Living off desalination is hard for him because to make that much water using ember he would have to constantly be there to make it. hours and hours sitting to watch liters of water boil only to get a couple of cups is excruciatingly numbing. Not to mention he also has to level up and travel to civilization if he wants food. He can get food in the ocean if he gets strong enough to battle the seamonsters or fish but he would need to cook it too which builds up more need for fuel or magic.
It is not cost effective as a long term solution.
There is better chances of making a well and boiling it then if need be.
Also since he has a magic inventory to store anything he can just store the dirt that falls on top of him. That and the shovel idea he has.