The French outlet names Everton and Benfica as declared candidates who have each conducted formal discussions about the 20-year-old, while Crystal Palace have made the same level of inquiry but are holding off on advancing any deal until they appoint their next manager following Oliver Glasner's departure.
A fourth club, a top-flight French side whose identity L'Equipe have not disclosed, is also in contention — with Lille having been interested as recently as January.
Maamma was named best player at last summer's Under-20 World Cup with Morocco — an achievement that significantly raised his profile despite his exclusion from the senior squad for this summer's tournament. The winger is described as having an explosive and technical profile that appeals to clubs looking for a player who could be bought and resold at a substantial profit.
Watford are expected to facilitate his departure. The precise fee has not been disclosed, though the combination of his U20 World Cup recognition and strong Championship season is likely to drive significant competition for his signature.
Palace's position is complicated by their vacant managerial role. With Frank Lampard the current frontrunner to succeed Glasner, any concrete transfer business is likely to wait until that appointment is confirmed.