There's what could have been done, and then there's what evidence shows was done.
No evidence has been found that I recall of clam shells or coconut shells being placed in the fire.
Heating water in a paper cup or coconut shell may be well known now, but was it known in 1937?
To do it, you have to know how to do it, and you need to have or make a suitable container.
If you don't have a container, you have to know how to make one and have the means to make one.
There was discussion of this on previous threads.
There was not much available in the way of survival training in the 1930s. The military got involved in it by way of WWII.
The main source of any such training (limited as it might have been) was the Scouting programs. I don't know what might have been taught in the Girl Scouts, but the Boy Scouts offered a fair amount of useful training (based on what my grandfather, a Boy Scout c.1910, and my father, a Boy Scout c.1939, have told me). But I also haven't seen any record that Earhart or Noonan had been Scouts. Nor that they had sought out any such training in preparation for the flight. Amelia was not big on thorough preparation -- even obvious things like learning Morse Code, or making sure the radios worked, and how to operate them. She wasn't even terribly careful about preflighting her airplane. It just wasn't in her nature to take the time to have survival training she had no intention of needing.
It's a reasonable question whether even an islander, stranded with no outside help and only the tools that have been found, could have survived for very long.
Christian has put it very well -- they were just not prepared for this. It's remarkable that at least one appears to have survived for more than a few days.
LTM,
Don