Stoop - no I have never swam in the sea at Calis - only in the pool in the complex where we stay.
I do participate in Open Water swimming (Salford Quays and Chatsworth House in the Great Swim Series and triathlons) and have scuba dived so I'm happy in water. I don't like the beach at Calis due to the stones and the heat and I'm not a lover of lying on a beach anywhere. I like being able to nip into AC to cool down. Salford and Derbyshire are cooler than Calis!
I cannot comment on what the water quality/ratio of turds is normally like in Calis as I haven't even paddled in it. It was only due to someone flagging the potential (recent?) situation whilst I was there (I got back home last night) that I took any interest in it's current state.
The colour of the sea did look different when it was pointed out though. It is a very distinct colour change that I certainly hadn't noticed before in 4 visits. It's not uncommon for Salford Quays or even Windermere to be closed to swimming due to Algae growth so large bodies of water can and do change for that reason.