Sailors have long known that if you lock the boat's prop so it can't spin, (say, by leaving the engine in gear) it has less drag than if you let it spin freely (say, by putting the transmission in neutral. The boat will sail faster with a locked prop than a free-spinning one. It has something to do with turbulence.

I do a lot of swimming and I've found I have the sensation of greater speed with my fingers spread open rather than cupped, but I never knew whether it was real or just a subjective feeling.

Maybe people should experiment by drilling holes in oar blades and canoe paddles.