Best fish locator is visual
Hi Dan...the only time that you need a fish locator is to know the depth for indicator fishing. The cone angle is to narrow for looking for fish, in 10ft. of water you might see 3ft. of bottom. Your eye's and ear's and a check out cast, will put you into more fish, if you don't get a grab then move untill you find them.
A side looker fish locator would let you look into the weed bed pockets.
"fish finder" s/b "structure indicator"
When I took the Phil Rowley’s seminar a few years ago, he believed in using the "fish finder" to inform him on how the trout "traffic" themselves in the water. Because of the trout's internal structure, I've been told by several company representatives (besides Phil) that these "fish finders" do not find trout, but you'll see pan fish or other types of fish.
Phil used the analogy of a mouse never scurries across the floor but follows the baseboard to go from point A to point B then to point C. Trout are very similar that they will drop into these bottom structure variations whileo searching for their food. That often the reason why they former stream bed is the best places to fish a lake. Using their indicators tells you where these slight breaks can be only 1-2' break but the trout will migrate through these avenues.
Differences between a trout and bass
I'm certainly not the one to answer this question but perhaps if you went to the black bass sites where they discuss their fish finders to the similar level that we trout fisherman discuss the finer points of wire dimensions for Copper Johns. A few fish finder sales representative have told me that something about an air bladder that exists for a bass that trout don't have which makes the difference of a fish getting picked up on the finder. Check those message board to get a more definitive answer.