I love to go boating on the Vltava River. I recommend Slovanka for boat rental, located right on Slavic Island under the National Theatre and Legii Bridge. Try a romantic evening paddle around Strelecky Island with fantastic view of Hradčany and Prague Castle.
Slovanský ostrov, 110 00 Prague 1
Open Monday-Sunday 11:00–22.00. Changes seasonally. Check website for details.