Like many words in natural language, "Swim" does not have a precise meaning. A dictionary will probably sat something like "Propel through water using limbs", although the word "swimming" can sometimes be used metaphorically, for example "I have bits of bacon swimming in my potato soup". Cleary, bacon does not have limbs!

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.