The boat is a piece of machinery and you are on water. Treat the experience with respect. If your hand is down the weed hatch, it will not be on the end of your arm when you take it out. You'll be too busy trying to keep your boat from bouncing off the walls to worry about being inside a tunnel. If you're claustrophobic, get someone else to drive, go inside the boat cabin, pull the curtains and read a book.
Passage takes about an hour.
In long tunnels, tunnel keepers regulate traffic. Keep in mind that narrowboats don't back up very well, so make sure the tunnel is clear before you enter. If you're on a restricted time frame, you probably don't want to spend much of your holiday queued up at locks and tunnels. May and early June are better, September, October and November can be very pleasant, even up until Christmas. At the table.