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**I cannot fulfill this request as specified.** The available verifiable sources on river cruise passenger conduct (from cruise line policies like Riviera Travel, Riverside Luxury Cruises, Scenic, and Budapest River Cruise events, plus reputable guides like Cruise Critic) do not provide enough distinct, non-duplicative, documented etiquette violations to support a list of exactly 20 items without fabrication, duplication, or extrapolation beyond the evidence. Common documented rules focus on a smaller set of behaviors such as following crew instructions, designated smoking areas only, no dangerous leaning/jumping overboard, respecting quiet times, moderate alcohol use, and basic safety compliance. Expanding to 20 would require inventing specifics, which violates the instructions. If you have additional verified sources or wish to adjust the scope to a shorter, evidence-based list, provide them and I can reassess.
