Remove the stems from the pineapples and discard. Peel off the skins with a sharp knife (keep the pineapple flesh in the fridge until needed). Place the pineapple skins in a large jug with the sugar and yeast. Cover with the water and stir continuously to dissolve the sugar and yeast. Cover the jug with plastic wrap and leave in a warm place to ferment for 24 hours.
Strain the fermented pineapple mixture through a clean muslin cloth (or cheesecloth) placed inside a strainer. Discard the pineapple peels and other solids. Either chill in the fridge until needed, or if taking to a picnic, transfer to plastic bottles (fill only part way) and freeze overnight. Take to the picnic frozen, and they'll defrost en route.
When ready to serve, cut the pineapple flesh into chunks and add to the chilled cooler along with the lemon slices and raspberries.