If an ESC fails short with the main power you’ll likely be much more concerned about the battery/power supply than the other ESCs. If the signal lines get shorted then the only risk for the other ESCs and electronics is undervoltage and a struggling power regulator, which isn’t great but shouldn’t cause major damage unless the ROV is in a sensitive location and floats/sinks into something it shouldn’t.
They’re quite robust, especially if kept within their specified operating range, and in the rare case of a failure it’s most likely to be something failing open, which just means that thruster will stop working. As a useful safety protocol, it’s worth regularly doing a quick motor check through QGC (ideally before each dive), so you can stop potential issues before they have a chance to become more serious.
You also tend to notice a non-functioning thruster pretty quickly if you’re using manual control - it throws the dynamics off quite a bit, and is much more rotatey than you get from just strong current or a tether snag.