...how then could a galaxy floating along in space make waves?
Nothing wrong with your argument, Paul. However, observations, as pointed out by Mike, inform us that relative motions of many galaxies within a given cluster are predominantly affected by their local gravitational fields (mainly by the dark matter in their vicinity), not by expanding space. What we see is many galaxies in various stages of collision. A prime example will be the collision of the Andromeda galaxy and the Milky Way
***At this point in cosmic history, the rate of expansion of space is insufficient to overcome those effects, but if the rate of expansion continues to increase, as predicted, then one can reasonably expect that collisions will gradually cease, galaxy clusters will be pulled apart and, eventually, each remaining galaxy will be its own 'island universe', with no evidence of other galaxies in its sky.
*** Up to 2012, there was no way to know whether the possible collision was definitely going to happen or not. In 2012, researchers came to the conclusion that the collision is definite after using the Hubble Space Telescope between 2002 and 2010 to track the motion of Andromeda. Such collisions are relatively common..."
http://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision
