The Water hyacinth and its ability to absorb pollutants from the water
Genes don't necessarily work in isolation (most don't). An ability to absorb pollutants would likely be a combination of many genes.

Also, what constitutes a "water pollutant" is very broad - you could be talking about pesticides, heavy metals, fertiliser, organic waste... If water Hyacinth absorbs all of these, it could just be the result of absorbing more water than other plants (with pollutants accumulating in the plants). Who knows - you should state your source of information.

In terms of how to figure out what gene(s) are responsible, you could consider a radioactive tracer to tag the specific compound (or even element) that you consider Hyacinth to absorb better than other plants. This might give you some idea of how it's doing what it's doing. For example, if you think it's absorbing a compound, you can tag that compound with a radioactive tracer, see where it's being accumulated (even it's path), and from there figure out what's happening.

The fastest path to figuring out which gene(s) are responsible from there would be to build up a list of all the genes you think might be involved in this path and create an array of cloned plants with those genes removed. If you remove the correct gene(s) from the equation, the plant will have less or no proclivity for absorbing the type of pollutants in question.