If you're using an HDMI converter then that's most likely your reason for Hydro Thunder not working. Hydro Thunder notoriously doesn't work well with VGA output (HDMI converters for the Dreamcast usually convert the VGA signal)
https://www.dreamcast-talk.com/forum/viewtopic.php?f=5&t=165...
To be honest I now pretty much just play Dreamcast games via the redream emulator on my steam deck and pc under my TV. Though obviously there are drawbacks with emulating. Every so often I bother to setup my Dreamcast with my ossc and play on real hardware.