It takes about 15 to 20 minutes to walk at a regular pace from HS to the YC. It takes about the same amount of time to ride the boat.
Keep in mind, if there is a line for the boat at HS, you may not make the first one and may have to wait for a second. Depending on the timing, you may be waiting an additional 20 minutes.
To be honest, my DH and I almost always walk. We've found we can get to where we are going before they ever drop off at the BC/YC docks.
I'd recommend making dinner for 8 or 8:30. That way you'll not be trying to rush around or worry about not making it. You'll also have ample time to make it back to the YC, whether you walk or ride.