Celebrating St. John of Nepomuk - patron saint of bridges, ship mongers, ferrymen and navigators - this once again annual event is celebrated (as it has been since the beginning of the 18th century) near the Charles Bridge on the Vltava River in Prague, Czech Republic. Due to St. John's widespread acceptance: patron of Bavaria, venerated by the Imperial family of Vienna; this is, once again, becoming a very important event in central Europe.
In its third year, an event celebrating boatmen and their boats. Originally for Venetian gondolieri, it is now open to most boatment and their boats, and most particularly to steamboaters. Although the closing date to register for special perks (like free accommodations and meals) with a steamboat is April 14, I'm sure the organizers will give some leeway. Contact them at: email@example.com or via the website below and mention the International Steamboat Society.
Even if you won't have a steamboat available, if you're going to be in east Europe check out the website at http://www.navalis.cz/cz/
and see if it's something you'd like to visit.