If you want to see good concentration, just watch a child play a computer. They are totally emersed in the experience.
When you are fully emersed, there's no mental chatter! You never find yourself in the moment, because once you realise you're in the moment, you're no longer in it.
I have a feeling that you are experiencing is fatigue. Yes, pool is not a physically demanding thing to do but the fact is that if you are cardiovascularly fit, it makes a huge difference in your mental abilities to maintain a performance level. Smoking, bad diet, inconsistent sleep pattern... etc can all be factors. Some types of energy drinks can also be used to help.