p. 405:

'....while Stroble and Hackett went over the side. [of the boat] About five minutes later, they came back up with a large trunk. Michael and Coleman lifted the heavy container into the boat. It was five by four feet and about three feet high. Coleman opened the hermetically sealed clasps and opened the trunk. [......] After closing the airtight trunk, he and Michael handed it back over the side to Stroble and Hackett. They took the trunk back down to the bottom and covered it with rocks.'

4 ft. x 5 ft. x 3 ft. = 60 cubic feet of buoyancy.

According to onlineconversion.com, that is 1699 liters, or equivalent to 1,7 tons of buoyancy, give or take a little for salt water. Even if this trunk weighed 150 kg in air, that would still leave more than 1,5 ton of buoyancy in water.