PPS (Prazska praoplavebni spolecnost) is major provider of boat & river cruise service. They run boats on the River Vltava during the summer months. Most frequent boats run to Troja, which is a northern Prague suburb. There are also river cruises around Prague from April to October (and limited cruises during winter) with food and drinks provided (cost around from 200CZK to 1000CZK). Besides PPS you can find some other small boats companies running their boats in summer. It is always good idea to book river cruise in advance as there are limited seats available.
 
Specail offer: Dinner on the River including tour guide

For younger people and teenagers, but not limited to, there are summer parties and disco boats operated by one or two radio stations. They’re popular in summer with live DJ shows. Entry free is around 100CZK. Journey take around 4 to 5 hours starting around 19:00 and finishing around midnight.

A bit of good advice. Everyone tells you not to trust the Czech taxi drivers especially to and from the Airport. Simple solution book and prepay your taxis through Prague-Airport-Transfers (they are on the web) It was less than £30 return and the service was superb. England, DEC 07 source TripAdvisor.com Most UK customers recommend Prague Airport Transfers.