i only thought of the pill answer after icot gave his answer.
mine was only a variation of his.
as to the barbecue riddle, i must protest
although you did say desert island, still, i believe using seawater was unfairly adding a new factor into the puzzle. now some riddles like the pill one have as their answer a ‘practical’ solution within the bounds of fairness, but riddles of jugs and liquid and other mathematical riddles generally have a purely mathematical solution, and although the answer you gave was correct, i dont feel in was in the mathematical riddle spirit.