Hello ourpeg and welcome to Rootschat.
The Inland Waterways and Docks Companies were a part of the Royal Engineers and comprised a good number of companies, so you need to know which company he was with. From there, you would need to find a war diary, which would be held at the National Archives.
In the first instance, I would suggest that you post his basic details: name and service number and if possible, a link to his service record, which you have presumably located on Ancestry.
Phil