I think you need to watch the videos a lil more closely 😃
They use sea water for cooking, so that serves the purpose of adding salt. Was shown in one of the videos from the very first week. I think it was Rajrani or Shilpa's video. Do check it again!
Yes, they did explore the forest/island for food. This week's videos show Tayaks discovering some sweet potatoes, but when they come for their task... Abhinav (An original Catan member) says they have been cooking Sweet Potato in different forms (Barbeque, boiled, etc) which clearly means they have been looking arnd for food at different places and have found different stuff to eat. It's just that we are not shown all 3-4 days worth of videos in 40mins! Also, in 2nd week's episodes Rajesh was shown hunting for crabs and snails. They were also shown boiling and eating them.
They did go fishing... We just remember the most recent footage or lets say love pointing flaws, but Shivam once said (In 2nd week's videos) that he went fishing with hand and caught few very small fishes. Sometimes it's just not possible to show everything, and it's not fair to assume and make remarks saying these contestants lack basic common sense! Maybe we all wouldn't fare any better when left on the Island like them 😃