On a budget, weigh it — Tiber River Cruise isn't cheap for what it is.
€15–35
Best understood as a sunset-and-aperitivo float rather than a sightseeing must — board near Ponte Sant'Angelo (Battelli di Roma run the main central service) for a glass of prosecco as you drift past Castel Sant'Angelo and the bridges in the golden hour. The one caveat to set first: the Tiber runs below tall embankment walls, so you see bridges and riverbanks more than the famous skyline. Come for the drink and the light, not the views.