Should not be... Dvt can be caused by many diseases as well as idiopathic (unkown cause). If u had DVT from, say trauma/surgery, and now perfectly fine and not on med, this should not be a problem. If currently taking blood thinner (coumadin (warfarin) etc..), your employer may restrict you. But being an active marine captain does not increase your risk of dvt. I would look into "requirement/exclusion" criteria. Good luck.
DVT and work. There are many people who have had a DVT and return to a normal life style and resume all of their activities that they performed prior to the DVT. I would doubt that a history of having had DVT would preclude you from obtaining a maritime captain's license. You would be wise to check with the licensing board to see if there are any restrictions placed on obtaining such a license.