Wow 400 € seems like a lot for one night at a best western, even in Venice. We stayed at a Westin property close to San Marco and on the Grand Canal for about $450. It was a couple of years ago but it was also June. The trick is to minimize the number of bridges between your hotel and where you hit the island. A private water taxi might cut this to zero but will come at a cost, about 100€.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016, Royal 2017
Star Clippers: Royal Clipper 2015