I would reach out direct to the hotel for questions about their services. This group is travelers who have stayed at this hotel. Personally, I took the train and then walked to the hotel.
They do not, but take the express train to central station, then take the train, to gamla station. Go left along the water and you are there. It is very easy. I am 60 and slow and had no problems at all.
Or take for about 12 EUR the bus to Stockholm's central station which is in walking distance of the ship. Takes about 1:15 hour.
Get an answer from Malardrottningen Yacht Hotel and Restaurant staff and past guests.