Which Schwalbe tires are your bike equipped with? The Marathon Plus is the most puncture resistant of the Schwalbe range, although there is also a slightly less puncture resistant Marathon (not plus). If you have the latter you may consider upgrading to the Marathon Plus model. They're a bit heavier but much more puncture resistant. These are the tires that many tourers swear by and ride many thousands of miles on.
As mentioned above, Slime inner tubes are an option, although I suspect anything big enough to puncture a Marathon Plus tire may be too big for slime or other sealants to fix, although having never tried it I don't know this for sure.
The canal path could possibly have something to do with your puncture, if there were lots of bushes/brambles nearby then a thorn or similar may have caused your puncture, or even broken glass that you may have not seen, however the fact that the path was gritty shouldn't have been enough to cause a puncture in itself.
Another thing to consider is that if your tires get too soft then this can actually cause punctures, as if you hit a large bump the tire deforms/compresses and traps the innertube between the tire and the rim, causing what's known as a pinch puncture.
Hope some of that helps!
