Ouch,
10 years ago I trode on one and i can still remember

!
Perfect day at Avalon, came in to adjust harness, stuck foot in weedy water BAM, felt like a hot needle.
The cobbler has 3 poison spines, 1 on top and one on each side.
I was struck on the side of the foot by a side spine as the bugger swam off.
They love to lurk in those weedy patches that form near the shore and are very common.
The hot water trick works but all you need is a good temperature increase not scalding.
The effects of cobblers vary from person to person.
Super Steve trode on one at Ledge point and still came fourth in the Ledge to Lanclein!!