I use floating plants that are intended for reptile enclosures to stop crickets or other live food from drowning in large water bowls, i cant remember which brand makes them but its one of the big companies.
i once had this problem. what i did was get meself some fishing line. i tied it to the plant and then tied the other end to a bit of lead plant weight, then bury this in the substrate. because the line is clear it works a charm!