I should say I'm talking about the effects of proper cold water here, 10 degrees celcius or less.

Without a wetsuit, I reckon you could give a few degrees either way and it would still be a bit unpleasant!

Going right back to the start;

"His heart jolted into another gear as the gasping cold sent his body rigid with shock."

Without a wetsuit, I reckon you could give a few degrees either way and it would still be a bit unpleasant!

Absolutely - this is how it feels to me at 10 degrees (or lower) scuba diving wearing a semi-dry suit! You know you're in trouble when you stop shivering... I have been in the sea (UK) on New Year's Day wearing just a cozzie. Now that's beyond words...

<Added>

... except the word 'stupid' obviously...

You're coming tomorrow aren't you, Katy? I'd love to to talk to you about this.

S

I am indeed - really looking forward to it too! See you tomorrow then! K

How about 'started'? (Although this has the problem of the more usual meaning of the verb.)

Or 'contracted with a start'?

Or 'jumped to a different pace'?

Chris