I don't think Swetha's choice was wrong. She could only choose members from her team for the elimination task and was left with Sana, Abhinav and Aastha to choose. If we look at the overall performances of Sana and Aastha, Sana is more stronger. Hence, Swetha selected Abhinav and Aastha for the stunt. She actually made sure that minimal amount of damage is done and Abhinav is also safe.
If Swetha chose Sana for the stunt it would have built an extra pressure on Abhinav coz Sana is good at water stunts.