Christy...
...you guys just need to look at where you want the sand from a gravity standpoint. If heavy rains are gonna wash the beds out, especailly towards the pool, then you might wanna 'contour' the earth around them to either channel it away from the pool or make like a 'dike' to keep the runoff contained within the beds...which means ideally the sand is not gonna sit on top of clay.
If clay, you'll be happy that you did if you 'condition' the clay with a permeable substrate. Home Depot carries what you need.
Once 'conditioned' the contained rain water will drain pretty good down into the beds, thereby helping the cannas.
They like a heavier phosphorus feed, the middle number (IE 15-30-15...that's N-P-K...Nitrogen-Phosphorus-Potasium...don't ask about the K, it's the symbol) maybe once or twice a month at label rates.
Now that I've said all that, Cannas love a lot of sun. So, scratch that idea. Unless you've pruned or lost a bunch of the cover you have, I don't think they'll do to good.
PS, you face the same issue with grasses. They like lot's of sun.
You could do some fun stuff with shade imatiens and the sand.
Impatiens flourish, not even you or Vrai could kill them, their are some dramaticaly bright colors to liven the dark feel I remember and...they're cheap. Tear them out in the fall.
Throw in some dracena spikes maybe or dusty miller and...viola.
If you wanna spend a few more bucks and get more towards that canna vision, check out some astilbes and ferns. They would look killer and, as perrenials, you do it once then maintain them.
Hostas maybe?
Pachysandra? (think care free)
Go to a garden center, tell them what you wanna do and describe the back yard, mainly how much sun and what time of day it get's it. Discuss the sand to.
You might go for some of those white architechtural pebbles to dress the top but sand should be fine for anything. Smooth peebles are pretty. Sharp ones keep the pigs and children out.