I agree with the PP, walk.
Once you walk through the International Gateway, you have about a 7 minute walk to the doors of the Yacht Club lobby. Captain's Grille is just inside the doors to the right.
Total trip, even at a leisurely pace, should take 10 minutes.
As for the boat, once you boarded at Epcot (which may or may not be immediate depending on if there is a boat waiting when you arrive at the dock, or not) you would stop first at the Boardwalk, then arrive at the Yacht Club dock, from which it is a short walk to the doors.
Total trip time would be in the range of 15-18 minutes, not counting time waiting for the boat at Epcot.
We generally walk into and back from the Boardwalk/Yacht Club/Beach Club areas to/from Epcot, just because we can control the time rather than having to allow for boats to arrive where we need them. It's a pleasant stroll along the beach to get to the Yacht Club.