1.0 is the fastest version of Fishy by at least a second, mainly because a gap in the final level was closed off in update 1.1.

EDIT 4/15/2020: The old way of downgrading using steamdb has been removed. You can still downgrade, however, using a third party program.

First you have to install Microsoft's .NET Desktop Runtime from this link: https://dotnet.microsoft.com/download/dotnet-core/3.1

Next, download DepotDownloader: https://github.com/SteamRE/DepotDownloader/releases

Open the command line (PowerShell for Windows 10) in that folder. In Windows 10 you shift + right click in the folder to bring up that option.

To download Fishy 1.0, type in this command and press enter:

dotnet DepotDownloader.dll -app 1147750 -depot 1147751 -manifest 5243640710114192967 -username INSERT YOUR STEAM USERNAME -password INSERT YOUR STEAM PASSWORD

Wait for the download to complete - it might take a while. It's worth noting that most of the files are contained in one very large file, so the download may appear stalled at a low percent completion for up to 25 minutes depending on your internet speed.

Once you have the files, follow step 9 and onwards of the old guide. https://www.reddit.com/r/Steam/comments/611h5e/guide_how_to_download_older_versions_of_a_game_on/

Quick guide of which update is which:

Does it have Endless Mode? Yes -> 1.2 No -> Does it have Options Menu? Yes -> 1.1 No -> 1.0