My normal AICW cruising speed is around 9knt which is about 4 yards/12 feet a second. My ability to stop the boat from 9knts to 0 takes about X seconds (I've never tried to time this, so let's just call it X for now). So, basically if I can see something that's 12*X seconds ahead, I'm good.
In 'normal' AICW water (depth 10-12 feet in channel), so if we use the 5:1 ratio ... I have ~55 feet to stop, so that makes my 'response time', X a little more than 4 seconds.
For the 8:1 example, I'd have ~88 feet to stop, or a reaction time of ~7 seconds.
Since main 'concern depth' ... water I try to stay out of ... is 6 feet and shallower, I'd think that even at my 'normal' cruising speed, and the shallow nature of the AICW, a forward looking sonar would be beneficial for places where I have concerns about the channel being affected by shoaling (pretty well documented for the AICW), or even more useful, when I'm really creeping along feeling out an anchorage area.
**NOTE, public math is hard, and I'm hoping I didn't make any calculation errors, if so, I'll blame it on the fact I just got back from a socially distanced happy hour.