Huntingdon Riverside Park.
Parking £2 up to 10hrs Mon-Sat and free on Sunday. No slipway charge but you should be registered for the river. The slip is sometimes gated/locked and you need a key which is £10 on top of registration fees.
You can turn right away from the slip and it's about 2mls before the next lock in Godmanchester, the other way is a similar distance to the old mill at Hartford which is National Trust and worth a look around.
Then just a few minutes back towards the slip from the mill is Hartford marina with the Hartford mill eatery where you can moor. It's one of the "two for £10" type places which we prefer to a posh place when in the middle of a few hours on the river.
So it's about a three hour outing to do the full length between locks inc a lunch at the mill eatery and nearer four hours if you visit the water mill itself as well.
Obviously rivers are a bit bland compared to the sea but I've lived adj to that stretch of water at various times for over 50yrs+ with punt/sailing/canoe/rowing/cruiser/narrowboat activities so it has lots of good memories.