If you’ve cruised the Dutch inland waters or the Baltic Sea you’ll have noticed how well the boat sails when you come out again. Your bottom is clean as a butcher’s slab at opening time because any incipient growth from the ocean loses interest in life as soon as it smells the sweet rivers. It takes a week or ten days for the full effect. Even if you aren’t lucky enough to visit such exotic locations it’s still worth knowing, because if you’ve a chance to lay up for a fortnight inside a lock you can offset the docking costs against the price of that midseason scrub you won’t be needing.