For creating the canals you might want to take a look at this mod https://mods.factorio.com/mod/canal-excavator and integrate it.

So instead of painting with the harbor dredging section one might use the excavator to create the canals for the ships.
And again when deepening the canals for the big ships.

added and update released

Nice and fast. Thank you :-)

A few improvement I have for you after playing around with it and getting it to work:

The excavator description says something about an placement preview. I didn't noticed one, however an indicator would be nice.

The orientation of the entity seems to be incorrect. North South and East West needs to switch, as the big box would like to stay on land and the scoop wheel should be in the water.
And the East West sprites are sitting too low, below their selection rectangle.

When the excavator finished, the water remains full of the decoration of the previous ground.

Not the end of the world, but maybe reduce the collision only to the excavation box (the 3x2 field that will not be excavated). So that one can replace an excavator you took by mistake and can place it down at the same position even with water under the wheel.

I like to play with alien biomes (https://mods.factorio.com/mod/alien-biomes), but was unable to excavate these tiles.

all fixed

works :-) thank you