I am curious if the ferry from Port Imperial to Midtown runs on Christmas day. Do the free buses from the terminal to across midtown run as well, in full capacity?
NY Waterway has published the holiday schedules on their website under http://www.nywaterway.com/AdvisoryDetails.aspx?aid=593. Specifically for your route you find the schedule at http://www.nywaterway.com/UserFiles/Files/2018/2018-12-25%20xmas/2018%20PtI-P79.pdf
In short: No service on the 25th December. Service until 8 pm on 24th.