Now even though Submarines (usually) have propellors, one can easily imagine a design that used limb-like oars to propel it. Does this mean that such a strange submarine is swimming?
The point Dijkstra was making is that the comparison is pointless: there is a fundamental difference between an organism swimming and a machine moving through water. Trying to equate these two actions is pointless, since it adds no new depth of understanding. He therefore dismisses the question of "can machines think" as having no usefulness.
I don't think that his statement is meant to be an answer, rather, I think it's meant to be a statement to jump off of. It's bringing up how definitions matter, and that in different contexts the answer to the same questions could be yes or no.
My intent in quoting was to point out how careless usage of words in AI can fool people, especially the layperson and create wrong expectations. Most can see the difference in the usage of word 'swim' w.r.t fish and submarines, but public knowledge about AI is not at the point where they can make a similar distinction when using 'think' or 'chat' between human and machine.
He uses it differently though, he's using that sentence to criticize Alan Turing's early hypothesizing about AI. If you're interested, that document linked has more context.