Historically speaking, timed fuses were far preferred to impact triggers for exactly this reason. Estimating the time of an arrow flight from the distance is a pretty straightforward affair, and if you're uncertain, you can just guess long; if the bomb sits on the ground for a few seconds before going off, it's no big deal. (It occurs to me that in a naval context, you might use airbursts, on the theory that showering enemy boats with flaming bits of wood and glass is better than missing. That would take some practice for sure, but no more than hitting a canoe from another canoe in the first place.)
For instance, the Wujing Zongyao, an 11th-century Chinese manual, suggests encasing the powder in a bag (of paper, cloth, or whatever's handy), attaching a fuse, and simply fixing the whole lot to the arrow and lighting it just before launch. Though less deadly than Greek fire or other impact-detonating schemes, it's far simpler and less likely to result in your troops lighting themselves on fire.