Stena run a ferry service from near my home at Harwich to the Hook of Holland, which is at the mouth of the New Waterway. From there its still I think a bus replacement service to Rotterdam due to the rail service suspended whilst a new metro/tram is built. Public cruises of Rotterdam harbour are run by "Spido"https://www.spido.nl
which can be accessed by a tram from Rotterdam central station Used to be number 5 last time I used it).
A small fast ferry runs from the Hook of Holland to Europort for passengers and bikes. This affords good views and photographic opportunities if you do a round trip which is very cheap around 5 Euros.
This link may help you access the Kiel Canal from Rotterdam https://www.rome2rio.com/s/Rotterdam/Rendsburg
best regards and best of luck and enjoy your holiday.