"She could", to me, has the added connotation of "if she wanted to". This is because we use 'can' in the past tense to indicate possibility (in place of the old English subjunctive). So if you want to stress that she had the ability to do it, say 'was able'. Because to me, "she could swim" also connotes a near-conditional meaning; "if [something happened], she could swim...". But both are right in the sentences you gave, it just depends on what you want to place the stress.