Slack water?

So I'm on my way to watch the Great River Race when I over hear one young man say to another "Yup, slack water, this is as high as the river gets."

I look over at Putney Pier and see the tide swirling by....hmmm.....!

Do I:

a) Approach the lads and say "You're talking total codswollop!"
b) Shake my head and go on my way wondering about the state of the British education system
c) Take a quick snap (above) thinking there's a blog post there somewhere
d) Something else - suggestions please