Is there any biological reason that professional swimming is dominated by white people?
It's not biological, it's simply an education issue, and googling would give you:

Just under 70% of African-American children surveyed said they had no or low ability to swim. Low ability merely meant they were able to splash around in the shallow end. A further 12% said they could swim but had "taught themselves".

http://www.bbc.co.uk/news/world-us-canada-11172054

Unlike the UK, where learning to swim is enshrined in the national curriculum except in Scotland, the ultimate responsibility in the US often lies with parents.