Our first river cruise is next November so I don"t know how things work. But I understand the coxktail hour is an a specific lounge and everyone goes there for drinks.
If no one from the ship announces dinner time during the cocktail hour, would it be out of line to stand up at the end of the hour and quietly ask the people sitting around having a drink in your area if they want to go to dinner now as a group? Then try to get a table all together when the group gets to the dining room. Everyone at your table would be on the same time schedule and easier for the waiters to serve.