I'm a but late to this discussion and likely the only one who really like the Keen Arroyo II sandals. They are the shoes I use most often by a large margin. The first thing I do with my new Keen's is to remove the bungee tensioning closure and replace that bungee with cord. This allows me to get a much more secure closure which keeps my feet from sliding sideways which was a frequent happening with the bungee and with other shoes with bungee closures. Loosening the cord a bit allows me to use wool socks or neoprene socks and still have a good fit. I really like the toe protection. I'm on my second pair of Arroyo's and they have been subject to much abuse. I've use them on local rocky, gravelly rivers to Shield Country portages with rocks and roots and water of unknown depth (and that's on the portage trails) containing shoe-grabbing roots and through muddy approaches with black sucking mud , etc., etc. The grip is probably not the absolute best but more than good enough for me. When paddling on lakes, I usually take them off in the canoe to let my feet dry as I find allowing them to get dry helps my foot-health. I find them very versitile.
As with most of you, I have tried a variety of footwear for paddling. I had settled on the Keen Newports but found they couldn't take much punishment before the attachments on the side at the back broke. The Arroyo II's are more sandal-like (more closed in) which is both a strength and a weakness. The more closed design helps keep most things out but does make it harder to remove things once they are inside. It's been my experience that all watershoes seem to get some debris in them and, yes, some are easier to get it out because of the larger openings so, whatever suits you.
Give them a go since they were free.