Via little ferries, or bacs, and the bridges between Rouen and the Seine Estuary
Cycling along the Seine à Vélo requires crossing from one riverbank to the other from time to time. Between Paris and Rouen, you cross via bridges or footbridges, in most instances with space reserved for cyclists.
Between Rouen and the Seine Estuary, many cargo ships ply the river, meaning bridges are rare. Rather than bridges, several free little ferries called bacs link the two riverbanks. Just one of the three great bridges between Rouen and the Seine Estuary is set up to allow bikes to cross the broad Seine without too many issues.
Remember, make sure to choose your final destination by the time you reach Jumièges. After this point, one branch of the route leads to Le Havre, north of the Seine Estuary, the other to Deauville, south of the estuary.