For romance, Vltava Pedal & Rowboats is hit or miss.
A cheap, slightly goofy, genuinely fun hour on the water — you steer your own boat past the swans with the castle behind you, and kids and dates both love it. The catch: it's purely seasonal and weather-dependent, the swan pedalos are kitsch, and current and traffic mean it's more 'drift near the bank' than a real voyage. Charming for what it is.