User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4775842-20120813183201/@comment-1758220-20120813185645

Look at how jagged and uneven his entire body is. I want you to take two giant cylendars into the water with you and try to use them as paddles, then come back and tell me how easy it was. In order to move quickly and efficiently through the water, the body has to be mostly smooth or at least not have a ton of obtrusive structures or textures so that the water can easily flow around it.

If you look at Lagiacrus, the plates on his back flow over top of eachother, creating a streamlined, layered pattern that allows the water to flow over it. Guran has tons of vertical spikes that point straight up, which would create tons and tons of drag when he tried to swim forward. There's also no fin/fluke-like apparatus on the end of the tail, just a fine tip, meaning that using it as a rudder would have little no no effect.