The idea of turning the UK into a Singapore-type of economy isn't new. Douglas Carswell (remember him?) wrote a book about it a few years ago.
From an economic point of view, fishing and agriculture only produce a tiny percentage of GDP. If you're sitting in an ivory tower and theorising, it makes sense to use our resources for services and knowledge businesses, which make a far greater contribution to the economy and, arguably, are a more efficient use of our labour and land resources. We can always import all our food from the US, where they have loads of land!
It's all nonsense, of course. Politically, it's totally unacceptable and Johnson must know that. He's going to have to backtrack, which will probably mean allowing more seasonal foreign workers. He's playing games, not only with the EU, but with his own hard line backbenchers and those who voted for him. All the time, he has to be seen to be delivering Brexit, to save his own position.