It's actually very difficult to get salt in the wild without any tools.
Even though he was right by the sea, with what looked like an endless supply of sea salt, the reality was much different. Seawater is completely undrinkable. Even the salt extracted through evaporation can't be eaten directly; it must be purified before consumption.
The process was time-consuming and a hassle, so he decided to save it for when he had more free time. In these early stages, while he hadn't lost much weight, getting by on the salt from the oysters was sufficient.
Although he was 'roasting' them, the oysters' high water content and naturally sealed shells meant they were actually being steamed.
The texture of the cooked oyster was completely different from when it was raw. It was something like seafood-flavored tofu, or maybe fish pudding.
