If I was about to run into a swimmer I think I would let go the back hand and carve upwind off the back foot as hard as I could then fall in a heap. For anything else I'd try to foot steer around it.
It just occurs to me that a catapult stops the board dead in its tracks so maybe a trip around the front would be in order. Not sure.

Yes, as Moby and Notwal say, heaps and heaps of pressure on the upwind rail, and particularly through your back foot, as if you're trying to do a radical tack. Points the nose of the board into the wind; stops you really quickly.

Gybing, on the other hand, will probably accelerate you, which is generally not what you'd want in this circumstance.

Good luck.

Many years I ago I hit a swimmer with my f'glass waveboard who was outside the flagged area of my then local beach. I was heading back to the beach behind the shorebreak and I was planing. It was like George St. that day with around 25 boards out, we were quite a distance from the flagged area, 18+ knots and around a 3-4 foot wave face and lots of whitewater and the sun was beaming. It was a really good summer sailing arvo. There normally isn't any swimmers outside the flags when we're out in those days, coz it would have been lunacy and akin to stepping blindfolded on to an expressway, most people have some sense and stay away.

Anyway, I was not expecting this, all I saw was this slim, grey receding hair, 60+ year old guy pop up out of the whitewater around tummy deep, remove water from his face with his hands and I'm around 2 metres away! Bang! The fear in his eyes when he put his hands down as he saw me coming still haunts me!

What saved his and my arse was I let myself fall backwards into the water as a brake, just as Mobydisc suggested, all I could think of was stop the board going forward! I didn't think of turning, not in that short space. I also hung onto the boom to make sure the rig stayed with me. It was enough to slow my board's momentum to hit him on the chest with a thud. It could have been much much worse if I hit him at full planing speed! Luckily my board was made at a time when a new movement saw surfboards, slalom and wave boards were designed with a blunt nose. If it had a pointy nose like my previous board I reckon he would have been badly injured or dead.

I felt like a karnt coz I thought I killed him. When I quickly came up from under my sail he looked at me in shock holding his chest. I thought he was going to have a heart attack as his eyes were just rolling around his head! I apologised profusely, he said nothing and went and sat on his towel on the sand. I think he was in shock. I went up to him, apologised again and asked him if I could take him to the surf club to be checked out. He said he was OK. In hindsight I should have got a clubby to come down to check him out. It worked out that he was OK...dunno about later though.

Anyway that put a damper on my arvo. I have a lot of respect for swimmers and they always have right of way, all it takes is one unfortunate occassion and you could have a disaster.