A few cats might swim, a few cats might like water, but non of the cats that I have ever homed have shown any great desire to get wet. They might lick their nether regions for absolute hours but that is not the same as actually getting wet.
Unlike a dog, a sensible cat won't bother to retrieve a stick, least of all one that is thrown into the river. If MM/John insists on taking Taxi afloat then may I suggest that lifelines and cat proof netting be fitted around the new Nyx. I suspect that a cat be fitted with a lifting harness but a lifejacket is probably a step too far, unless John wishes to be clawed to death! A cat overboard drill would be a wise precaution, perhaps with a large landing net near and handy. Perhaps dunking Taxi into a well filled, cold bath will give John the answer to his question.
I don't envy Taxi his potential adventure afloat, or his dunk in a cold bath, brrrrr!