I have thought of a level that involves swimming as well. For now I'll consider it a complete level in my level-design project, but I could always put more things on in the future. Note that I had built this level on the "Milestone 1" 2003 version of SuperTux (on my Linux system) and then ported the code over to a different computer with the current Development Version of SuperTux (on a Windows 7 system). Note that the level height is still only 15 blocks like in the original version, so magnify the level before you play it or put it in a short-height window! I took advantage of well-placed hidden blocks at the water line, a lowered the gravitational rate, and specially thought out graphical design all in tandem to put the level together.
In any case, it may provide a few ideas. It just seems to flow well for some weird reason.
I find it interesting that mteufel mentioned the
https://github.com/SuperTux/supertux/issues/217#issuecomment-233174252 article, I had no idea that it was even considered that Tux was to swim in the actual code. Either the code I am looking through was too out of date
(most likely) or I just hadn't seen it in the code yet. The only source code I have available on either computer is the old Milestone 1 code, after all!