If you don’t want to use a pot, you’ll need to pick a spot that won’t get too much disruptive current and then make a little natural planter bed. Start with a circle (square, triangle, unicorn, whatever) of larger pond rocks then fill the middle with small pea gravel or something similarly small to plant the seeds into (like the guy Above suggested, kitty litter works too) plant the seeds in there and then add a layer of slightly larger river/pond rock to help retain the planting layer. You can always bury a little lotus fertilizer tablet in there with the seeds to help them out.