Batavia

If you are serious about obtaining an excellent seal, then use a cable transit system, such as this: https://www.powerandcables.com/product/product-category/roxtec-crl-cable-transit-frame/ Sit down before you start looking at prices! Edited to add: For single cables, just use plastic stuffing glands - cheap (in small sizes) and effective.

Not so much guessing if you use a Fuller's calculator - the scale is 500 inches long!

Selway Fisher have a design for a narrowboat (and a wide beam!)- and have produced other designs in the past. https://www.selway-fisher.com/Mcover30.htm#AVONCLIFF NARROW

I normally use rigid uPVC conduit. For 12 volt circuits I have just drilled holes into the conduit, fitted them with grommets and brought the wires out directly to fittings that are close. For 230 volt circuits, I use normally use singles and proper conduit fittings throughout.

16mm LDPE pipe is widely used for irrigation systems and compression fittings (in addition to useless barbed ones) for it are readily available. No idea how its wall thickness compares with the boat piping.

No, I am Chris! Frank has kept his various boats there for decades too.

Correct! You get the paved area fronting the canal and a microscopic garden round the back. Your recollection is correct! We moored our boats (2 x Daedalus + Batavia) there, on and off, for 40 years.

No, its the house which was built next door about 11 years ago, on the site of the old boat shed (which was partially burnt down by a local juvenile arsonist after the boat hire business closed).

A little place for sale on the GU in Berkhamsted. https://www.rightmove.co.uk/properties/137692502#/?channel=RES_BUY

Try Aquadag!

We had one like this many years ago and the previous occupier had managed to break off the head of the bleed screw flush with the radiator, with the result that it was more or less invisible.