Floating Bayshroom
When some Bulbus salmacidus developed a mutation that caused their roots to not properly attach to the floor by not making an effort to anchor it, they were meraciously able to survive as they would float up at the surface and they would be able to collect sunlight for photosynthesis with the extended bulbs, even if toppled over. The roots got the nutrients from the water. Over time, their stems shortened for more efficient and stable floating to maximize the sunlight collected, and they evolved into a new species, Bulbus semifloatus.
Bulbus semifloatus is only fifteen centimeters (15 cm) tall due to that, but at the surface a large stem is not important to them and they float better with a shorter stem. The resistance of the twenty-five centimeter (25 cm) bulb keeps the bulb afloat at the surface while the stem and the roots are under the surface. The roots grow firmly outwards to stabilize even further, so this polyph is rather hard to topple over with currents alone. Seeds grow on the tendrils like in their closest relatives, which still sink down to grow new polyphs, in which some will successfully float up after being carried by currents.
These will almost typically be fully mature at three (3) weeks of age, so they can reproduce quickly to make up for the ones that don't get carried to the surface in time or get eaten. Most of the time, they do eventually make it to the surface before dying as their flat-out roots are in the right shape to drift upwards and not downwards, and the Bulbus semifloatus are of a similar density to water.