MsLynn
Try performing a Search for "aquatic" or "aquatic life" from your Roku Home Screen and see if it is available as a channel/app install instead of a screensaver for your Roku 3.
On some RokuTVs and devices you first have to install it as a channel/app and then you will find it under your Settings/Themes/Screensavers as well as located amongst your channels/apps on the Home Screen. (means you can also launch it whenever you want from the Home Screen).
On other Roku devices, it is simply a screensaver. (no seperate channel/app install required).