They will keep sticking their heads under the water ! :-)
"He had softly and suddenly vanished away---For the Snark was a Boojum, you see." The Hunting of the Snark, Lewis Carroll
Was it the common Teal you saw Cath or the Green Winged variety?
I'm a bit new to this, so I'm assuming I've only seen regular teals.
Here's a photo from the Usk in Newport today with a couple of drakes that look different to each other, but maybe they've just fluffed up their feathers differently. One of the birds seemed to be making calls like a referee's whistle, but I wasn't sure where the sound was coming from.