Hi Mike, They both mean the same thing.The first might be more formal. In the first sentence you use "infinitive+koto+ga+dekimasen" whereas in the secon sentence you simply use the potencial form.

Hi! They're both used to express "can't swim". 泳げない is more formal than 泳ぐことができない。 Example: "I can't swim in the sea" --> 海で泳げない　／　海で泳ぐことができない。