For captains of merchant ships and fishing vessels, Danish or EU/EEA citizenship is required.
"§ 13. Those in § 6, para. 1, § 7, para. 18 pcs. 1, § 9 and § 10 stated requirements for Danish educations and diplomas do not apply to persons who meet the conditions in the European Communities directives on the introduction of general schemes for mutual recognition of diplomas for vocational qualifications, cf. Act no. 476 of 9 June 2004 on access to certain professions in Denmark. Masters of merchant and fishing vessels must, however, have Danish citizenship."
(Ship Crew Act (No. 74 of 17/01/2014), last amended by Act No. 400 of 02/05/2016, § 13 [google translation])
Ship Crew Act (No. 74 of 17/01/2014), last amended by Act No. 400 of 02/05/2016, § 13
[<a href="https://www.retsinformation.dk/eli/lta/2014/74">Act</a>]