I bought the Hook N Line SD card for my Garmin 94SV. It runs good on my machine and shows lots of reefs. Some probably don't exist anymore, but it will get you around the lake. It's still just a map. It won't guarantee you catch fish, so I don't know if I can comment on your question, "Do they really work?".
https://hooknline.net/t/hooknlinesdcards