It is amazing the things that you can find on the internet. This sport was started about 15 years ago to to bring attention to Llanwrtyd Wells, the smallest town in Great Britain. The contest consists of a 60 yard trench that is dug into the bog, which fills with water from the surrounding area. Contestants swim two lengths of the trench using snorkels, masks and scuba fins. Some people wear funny outfits to add to the hilarity. Proceeds from the event are donated to a charity.
I think that this is a super crazy activity. If you want to experience the wildness of the event, below the fold is a YouTube Video from the 2007 Bog Snorkeling Championship in Wales.
As a biologist I don't think that a 60 foot long trench is very good for the health of the bog. However this sport is only held once a year so it is probably not making a hugely negative impact on the vegetation. They might even use the same trench from year to year. Recycling at its best.
There is also Mountain Bike Bog Snorkeling and a Bog Snorkeling Triathlon. I have included the links if you are interested to learn more.