We went to Rouen by train from Le Havre and it was very easy and much cheaper than an Oceania trip.Just google SNCF for details.
Alternatively Honfleur is lovely and all you do is get a taxi outside the terminal,make sure you book a return trip.The first time we did this we didn’t and had a few anxious moments because we couldn’t find any taxis in Honfleur,but got over it