Recently, waterway=pressurised was reviewed on osm wiki, encouraging the use of waterway=* key to map not only free space but pipe flow waterway features also, with content (waterway) vs containers (tunnels, pipeliens...) distinction
​https://wiki.openstreetmap.org/wiki/Tag:waterway%3Dpressurised

It may be good to add some new waterway presets as follow on iD:
==Penstock==
waterway=pressurised
man_made=pipeline
substance=water
location={underground,overground}
usage=penstock
name=*
operator=*
diameter=*
material=*

==Pipe flow tunnel (different from canal, canal is always free space - even underground) ==
waterway=pressurised
usage={headrace,tailrace,transmission,irrigation}
location=underground
tunnel=flooded
name=*
operator=*

==Canal preset should be updated as follow==
tunnel={culvert,flooded}
usage={transportation,transmission,irrigation,headrace,tailrace}

Main validation rules :
usage=penstock requires man_made=pipeline and waterway=pressurised
tunnel=yes and waterway=pressurised requires man_made=pipeline
tunnel=flooded is highly improbable with man_made=pipeline
waterway=canal shouldn't be used with tunnel=yes
waterway=pressurised requires tunnel=flooded or man_made=pipeline

It may be possible to render pressurised waterways with grey or white casing depending it is inside tunnel or pipeline.

These are all suggestions to make presets more comprehensive. Feel free to implement part of it only.