I'm not sure it's entirely irrational to fail to predict what the government would eventually settle on, especially when said government was at least feigning interest in getting a better deal for fishermen...
Particularly not compared with some of the other Brexit vote rationales
What the fishing industry in the UK wanted out of Brexit was pie in the sky though. It's not like the plan was to saw the British isles off the continent and set sail into the Atlantic, so they still had to deal with the fact of fishing in commercially contested waters.
It seemed like their plan was to be able to do more fishing of the kinds of fish that are available around coastal Britain that aren't popular in the UK and sell them to the Europeans. I say plan, I mean "barely thought out idea". Because the reality is that the ships that do that fishing already exist and already have agreements with the processing businesses, which are already in the EU. And fishing is such a tiny part of the UK economy in comparison to some other countries that access was just traded away.