Hello,
I checked with a hex editor and didn't find any unusual character. The csv can be cleanly imported into libre office. Your test file works with subsurface.
When I add as first line this:
Time (sec),Depth,Water Temp
it stops working.
I removed this first line from the csv and it works.
Thanks for your help, it seems subsurface doesn't like column headings.
The issue is resolved for me.