What's the longest boat ride you've ever been on?
Plymouth (SW England) to Santander (Northern Spain) which takes about 11 hours. We did the overnight crossing and got a cabin. This route has
Some people do the return trip as a mini-cruise, spending a just a few hours in Spain for lunch and shopping. Santander is a typical shit-hole ferry port and the ferry itself ain't that great, probably one step up from the south coast-northern France ferrys. You've got to be fairly cheap-scate or desperate or sad to want to do that trip as a mini cruise.