What could cause sudden deafness and deliriousness on a boat adrift?
He probably had hypernatremia: salt toxicity from drinking seawater.  That definitely can cause delirium.  It was harder to nail down that hypernatremia could cause deafness specifically but I found that for pigs.

http://www.merckvetmanual.com/toxicology/salt-toxicity/overview-of-salt-toxicity


  Affected pigs may be blind, deaf, and oblivious to their surroundings;
  they will not eat, drink, or respond to external stimuli.


It would be easy to think a person was deaf if he were altered / delirious / encephalopathic.  His brain does not process what he hears and so he is oblivious - like these pigs.