John B and Sarah have Superhuman Strength

Doing some basic math: assuming the backpack John B had was about 40 liters, and let's just say it was half full with gold, that would make it 20 liters of gold in volume. With the density of gold at about 20 grams/milliliter, that makes it 20 kilos per liter. Multiply that by the volume and the bag weighs *400 Kilos*. (about 900 pounds) - and this is generous.
I find it very impressive that they were able to swim that bag through the underwater tunnel and run around with it. This was probably the most supernatural part of the episode.