All other placeable items in the game can be selected (held in the hand, if there are any in the inventory) by hovering over an already-placed instance of the same item and pressing Q (I think that's the default). For example, hovering the cursor over an assembler and pressing Q with an empty hand puts the stack of assemblers in the inventory into the hand.

This behaviour doesn't work with waterways. When I place the cursor over a waterway and press Q, nothing happens whether or not I have waterways in my inventory.

yes. thats because the waterway item in your inventory is not actually the same entety as a placed waterway. this is a result of conflicts between the intended behavior of this mod and engine limitations of a game that never actually was meant to have any buildings on water. i probably could fix this, if its considered a big deal by players. do you consider it a big deal?

It's the sort of thing that's completely dependent on playstyle - I never use the toolbelt hotkeys, if I'm building by hand I do almost all item selection using Q so it's quite jarring when it doesn't work on waterways.

On the other hand, I very rarely need to select waterway items - I can select it once, place a few thousand, then not have to worry about it for days. So I'd call it nice to have but not a big deal.