Brob On Toka

Here is another logical yet significant deduction about the inhabitants of the superjovian planet called Brobdingnag by human beings. It is OK for one of their number, called "Brob" because no human being can pronounce his real name, to spend weeks at sea with the British fleet on Toka because:

"Brob had eaten before they left Mixamaxu, and one of his nuclear meals kept him fueled for weeks."
-Poul Anderson and Gordon R Dickson, Hoka (New York, 1985), p. 200.

Consuming isotopes, he does not need to eat two or three times a day.

Needless to say, Brob's role in the story is more than just either light relief or a means of quick transportation for Alex Jones from Earth to Toka. Like a superhero, Brob is powerful enough to defeat an enemy without either suffering or inflicting any physical injury. As he swims towards the French fleet, their cannonballs bounce off him. Catching a mast with a hooked chain, he dives and swims until he has overturned the ship enough to soak its gunpowder. When he has done this twice, the remaining ship flees. Thus, a battle has been prevented with no harm either to the enemy or to their single adversary, just as Superman would have done it.

My only question here are:

Surely Brob should be so heavy that he sinks like a stone instead of being able to swim?

Why can I not logically deduce in advance whatever is the next contribution that the authors will have this character make to the plot?