Prague Vltava River Cruise

I took several river cruises in all those years visiting Prague and cannot answer your question which one is the best. They are pretty similar. You don't have to book it ahead. Once you are in Prague go to several visitor centers and ask for their opinion. You can also try google.

I have done the Little Venice boat tour, was not impressed.
there is a boat that runs from Naplavka to the area near the Zoo but only runs until near the end of this month.
http://www.praguesteamboats.com/
never felt the need to do an evening cruise .Plenty of static boats on the river that you can have a meal on without having to cruise.

Rent a paddle boat from the little island in front of the National Theatre and spend an hour bobbing around, propelled by your own peddle power. It's wonderful, and you can even pull up alongside a riverside pub where they'll hand you beer into your boat. Boat rentals continue right into the late evening - they simply give you a little oil lamp to hang on your boat and then off you go. It's the best way to experience Prague from the river.