Wide relates in the end to drag. And yes the number of rowers matters -- a bigger ship requires more rowers, thus more mass etc. Generally, there is the "optimization problem" with each such system that a bigger vehicle requires bigger engine and more fuel which means more mass. But here they mainly refer to the ratio of length versus width and from my and above posters guess they refer to the profile of the boat in the water.

Edit: to amend and clarify: mass in a fluid affects displacement (assuming you stay above surface) and thus drag and maximum speed. Here is a crude reference about boat speeds https://www.boats.com/reviews/crunching-numbers-hull-speed-b... -- if I get time I will do more Google fu but I am pretty sure anyone can find the complete derivation online.