Solved by Michael Josephson

solution: more hat puzzles

Fisherman C shouts out.

Fishermen A and B are in the same situation - they have no information to help them determine their hat colour so they can't answer. C and D realise this.

Fisherman D can see both B and C's hats. If B and C had the same colour hat then this would let D know that he must have the other colour.

When the time is nearly up, or maybe before, C realises that D isn't going to answer because he can't. C realises that his hat must be different to B's otherwise D would have answered. C therefore concludes that he has a black hat because he can see B's white one.