Is there some open dataset with maximum swimming velocity and related information about size, for whales or fishes, which can be used to illustrate Froudes law?
There are a number of research papers/study on this subject.
This 1991 paper covers a lot of historical as well as data current at the time.
Fish swimming stride by stride: speed limits and endurance, Videller, 1991 http://www.researchgate.net/profile/John_Videler/publication/226033193_Fish_swimming_stride_by_stride_speed_limits_and_endurance/links/0fcfd506672ef8c64f000000.pdf
Here's a 2012 paper on sharks in Greenland
The slowest fish: Swim speed and tail-beat frequency of Greenland sharks, YY Watanabe, 2012 http://www.researchgate.net/profile/Christian_Lydersen/publication/258980090_The_slowest_fish_Swim_speed_and_tail-beat_frequency_of_Greenland_sharks/links/0deec52988c6c8cd91000000.pdf
Here's another paper with data, circa 1990 http://pubs.iclarm.net/Naga/FB_1363.pdf
And here's another paper source: Morphological predictors of swimming speed: a case study of pre-settlement juvenile coral reef fishes, R Fisher, 2007 http://jeb.biologists.org/content/210/14/2436.long