Going to Phuket at the end of December. What areas with bioluminescence close to Phuket can one swim in at night (Krabi is relatively far)? I found a tour operator (John Gray) but they only casually mention bioluminescent plankton, and not as a key point of the experience.
Maya Bay Tours offers a Bioluminescent Plankton Snorkeling Tour of Maya Bay, departing from Phi Phi Don. They also land on Maya Beach in the afternoon, after the crowds are gone.
When it’s dark, you will have the opportunity to snorkel the amazing bio-luminescent plankton. See the incredible underwater light show, right in front of your eyes. To get the most out of this tour you need to go in the water with a mask on. You can see the bio-luminescence sitting on the boat or by dangling your feet in the water and kicking. However if you hop in the water with a mask only then will you see the psychedelic light show at its best.
Phang Nga Bay sea caves tours that go there at night (e.g. John Grey's Hong by Starlight) also offer an opportunity to see the bioluminescence. I took this tour in Dec 2016 and the experience was quite magical. In the caves, the guides try to avoid turning on their headlamps, so the guests the see the bioluminescence and the kratongs. You can't really swim there because the water is very shallow, but if you dip your arms in the water, there's quite a show. Once outside the caves, you can ask the canoe paddler to swim near the boat (typically they speak little English and won't say no), though the guides on the boat may tell you to get out of the water due to jellyfish. The boat itself must also stay lit, which will ruin the bioluminescence.
According to another tour operator, the bioluminescence can be seen year-round, and the plankton lives all around Phuket as well. So other than these two tours, one could look at a diving boat liveaboard tour and ask about night swims with plankton, or at chartering a boat that's equipped for night time excursions.