Interesting story at stuff.co.nz about how footwear may be contributing the spread of the invasive freshwater alga known as didymo (Didymosphenia geminata, also known charmingly as "rock snot"). Apparently the felt-soled boots that have become popular among anglers are impossible to clean or disinfect in a way that assures they are algae-free. The cleaning products that anglers normally use cannot penetrate the felt properly, and the felt stays wet for a long time, allowing any embedded algae to survive.
While there is currently no legal regulation outlawing the use of felt-soled boots, freshwater anglers are being urged to switch to foam-soled ones that dry out faster and are easier to clean. As far as I can tell, the advantage of the felt soles is to prevent slipping, but IANAF so anyone who knows for sure should feel free to leave a comment.