The Hornets failed to secure promotion back to the Premier League last term and are looking at another season of Championship football.
Winger Sarr has long been linked with finding the exit door at Vicarage Road in multiple transfer windows and he may do so this summer.
Marseille have zeroed in on Sarr and are keen to take him to Ligue 1, with their opening move having been made.
They have, according to French outlet Foot Mercato, sent an initial offer, a verbal offer, to Watford for Sarr.
Watford are suggested to be fully prepared to cash in on Sarr this summer and are looking for between €22m and €25m to let him go.
Joining Marseille would mean a return to France for Sarr, who kicked off his career at Metz before then signing for Rennes.
A move to the Stade Velodrome would also hand Sarr an opportunity to play Champions League football next term, with Marseille having finished third in Ligue 1 last season.