Some helpful replies, some less so (!).
I appreciate the point about keel coolers possibly being more vulnerable to damage, but in the event of a total failure the engine would then be directly sea water cooled until repairs were made, and if you've hit anything hard enough to damage it you might have more urgent problems anyway...
Regarding the issue of internal corrosion, this wouldn't be an issue if the keel cooler/skin tank wasn't steel.
Prv, sorry wasn't meaning to disparage direct cooling... it's a perfectly good idea if your engine isn't steel or cast iron and can tolerate chronic scaling of coolant passages.
I knew it was time to suspend the sailing lesson when the forked lightning hit the Loch surface!!