Step 1: Prepare your serving cups. You'll need about 8-10 clear plastic or glass cups (8-ounce size is ideal). Make sure they are clean and dry.
Step 2: Crush the graham crackers. Place about 10-12 graham cracker sheets into a Ziploc bag and crush them using a rolling pin until they resemble fine sand. Set aside.
Step 3: Prepare the blue Jello. In a large heatproof bowl, combine 2 (3-ounce) packages of blue gelatin mix with 2 cups of boiling water. Stir constantly for at least 2 minutes, until the gelatin is completely dissolved.
Step 4: Add cold water. Stir in 2 cups of cold water into the dissolved gelatin mixture.
Step 5: Chill the Jello (partially). Place the bowl of liquid Jello in the refrigerator for about 1.5 to 2 hours, or until it is partially set but still liquid enough to pour easily. It should be the consistency of a thick syrup, not completely solid. This partial setting is crucial for creating distinct layers.
Step 6: Create the 'sand' layer. Spoon about 1-2 tablespoons of crushed graham crackers into the bottom of each prepared cup. Press lightly to form an even layer.
Step 7: Pour the first Jello layer. Carefully pour the partially set blue Jello over the graham cracker 'sand' layer, filling each cup about 1/3 of the way.
Step 8: Add the first layer of 'fish'. Place a few gummy fish or other candy critters into this first Jello layer. They should sink slightly but not fully touch the bottom if the Jello is correctly partially set.
Step 9: Chill again. Return the cups to the refrigerator and chill for another 30-45 minutes, or until this layer is firm enough to support the next.
Step 10: Add the second Jello layer and more fish. Once the first layer is firm, carefully pour another layer of the remaining partially set Jello, filling the cups about two-thirds full. Add more gummy fish and candies, positioning them to look like they are swimming at different depths.
Step 11: Final chill. Return the cups to the refrigerator and chill for at least 2-3 hours, or until the Jello is completely firm.
Step 12: Decorate (optional). Just before serving, you can add a few more gummy fish or a sprinkle of edible glitter on top for extra sparkle. You can also add green gummy worms or candy strips for 'seaweed' if desired.