De Telegraaf report that other Premier League clubs are interested in the 20-year-old custodian, but Watford appear to be the most attentive.
Eredivisie clubs are also taking note of the youngster, who earns a modest salary and has 18 months left on his current contract.
De Boer has played 19 league games for the second division club this season and kept six clean sheets, including three in his last four starts.
Watford are understood to be on the lookout for a younger keeper to act as back-up to Heurelho Gomes, but it might be too much of a risk to go for one as inexperienced as De Boer right now.
However, the Netherlands Under-20 international could be one to keep an eye on for the future.