They don't have to exchange signals if they're just passing one another, and that was a question I just answered! They should exchange signals if they need to alter course while passing one another starboard to starboard, for example.
Deck
Rules of the Road: INLAND ONLY When shall whistle signals be exchanged by power-driven vessels in s...
Discussing this exam question.
Rule 34 (a), (I) and (ii)... I was sure it was wrong....