Good luck! Take an Airzound for the fishermen!
tbh if you have the whole day you can get in some decent miles before anything is liable to get too busy and not be frustrated by it. At least if there's folk about people watching can liven up the ride.
In terms of being busy on nice summer days, I think the worst section by far is from when you hit Watford, thru Rickmansworth and up to around Maple Cross. There's plenty of permanently moored barges around here and people just seem to like hanging out by the canal on nice days (god knows why - I wouldn't find it relaxing to sit next to a busy towpath). It's not unusual to have people BBQing, sat round in folding chairs, etc. The good news is, should you wish, you can quite easily bypass this via the A421 and pick up the canal again after Maple Cross. The A421 isn't exactly scenic, but it's not too bad either. The canal should calm down after that.
Can you advise where we can join the A421 to avoid this part, and where do we re-join the canal? Thank you.