We had heard about the sharpie as well. When we did our haulout in December we polished up the feathering prop and applied 4 coats of sharpie texta. One application each hour to get a good coat.
It lasted a week
I think we can safely say 'myth busted!'
But worth a try
Our next attempt will be the thick lanolin grease applied underwater. Got a tub of the stuff and we're just waiting for a break in the weather to get out to the reef where we're not likely to get bitten in the **** by a croc.
We'll let you know how that goes - the grease - not the croc