This may be a better way, instead of changing the Media Kind of a podcast episode to make it a song. It uses the shuffle's features as designed. First, if the iTunes sidebar (along left side of window ) is hidden, from the menu bar, under View, select Show Sidebar.
Create a new playlist by clicking the Plus sign at the bottom of the sidebar, and name it as desired (this playlist will hold the songs and podcasts for your swim). Add the songs you want for your swim from your Music library, and then add the podcast episodes your Podcasts library.
Select the iPod in the sidebar under DEVICES. To the right, the iPod's Summary screen is shown. Toward the top, where it says Summary, click on Music next to Summary. This is the iPod's Music screen. Check the box for Sync Music.
NOTE: If your iPod is currently set up to use the Manually manage music setting, setting up automatic syncing replaces the iPod's existing content. Any songs that are on the iPod that are not in your iTunes library (on the computer) will be lost. If this is a problem, please post back. It's easier to explain using automatic syncing, but you can also set this up using the manual method.
Choose the option to sync Selected playlists, artists, albums, and genres. Find that playlist for swimming under Playlists and select (checkmark) it. If you have other playlists with songs for the iPod, you can select them too. You can also select other items from the other lists, as desired.
Toward the top where you clicked on Music, click on Podcasts. This is the iPod's Podcasts screen. Check the box for Sync Podcasts (and uncheck the box for Automatically include... if checked). You don't need to specifically choose podcasts here. Just go the bottom of this screen, and where it says Include Episodes from Playlists. Find that playlist for swimming there, and select it.
When you click Apply, the items in your selections (on both the Music and Podcasts screens) sync to the iPod. Going forward, you can update that playlist for swimming (add/remove/reorder items) in your iTunes library. The iPod does not need to be connected. The next time you connect the iPod (or click Sync if already connected), iTunes updates the iPod with the same changes.
When playing the shuffle, use VoiceOver to select that playlist for swimming. The shuffle's power switch needs to be in the (middle) play-in-order position because podcasts are skipped on playlists when the shuffle plays in shuffle mode. The playback order is what you see in iTunes, before syncing. So, if you want to make sure podcasts are always heard during the swim, you can put them first in the playlist order, followed by songs. If you want a "random" selection of songs to follow the podcasts, I would use the Genius feature in iTunes to create a list of "sound good together" songs based on a particular song that you like; copy that song list to your swimming playlist after the podcast episodes.