Rotterdam on April 26, 2019 from 7 AM - 6 PM

I can't imagine that doing Keukenhof on your own would save you time. It's a good half hour walk from Wilhelminkade to the train station, and there's no direct train, so you would need to do a train/bus combination to get to Keukenhof. (Of course, you could take a taxi to the train station.) You could get an earlier start than waiting 3 1/2 hours for the ship excursion to leave.

Otherwise, 7am is awfully early, and many things won't be open, and it may be a rush to do much between 3:30 and 6.

If you figure out how to enjoy some time in Rotterdam, here are a few suggestions:

1. Visit the Markthal (Market Hall) and sample some of the foods available there. Also in that vicinity are the famous cube houses, including a furnished model home you can visit, and the stunning central library.
2. Stroll around Het Park (The Park)
3. Visit the Museum Boijmans van Beuningen (https://www.boijmans.nl/en) or other museums in the area (Museumpark).

Deduct 2 hours at least from the time you have available. You will not get off the ship until after 8:00 and will be told to be back on board by 5;00 if not 4:00. If you google "shore excursions in Rotterdam" you might some ideas on how to spend your time.

As an alternative to Keukenhof you can do a daytrip with the waterbus to the windmills of Kinderdijk. The waterbus departs just across the Erasmus bridge next to Wilhelminakade.
https://www.waterbus.nl/en/about-the-waterbus

Consider a boat tour of one of the world's great ports.

https://www.spido.nl/en/tochten-cruises/rotterdam-havenrondvaart

Boijmans museum will close sometime next year for renovations (that will last 6-7 years).