Scott, I've seen more than 1 video, right here on 2cool, of people trolling those **** pops and live mullet for tarpon here in So. Texas. For decades, the WR tarpon on 20 lb., was 243 lbs. and was caught trolling in Key West harbor. Capt. Bob West used one of those Atom swimming plugs with a single hook in the rear, intead of the treble.
I believe the key is water depth. Key West harbor is dredged to 32 ft. That water you mentioned in Nicaragua is pretty deep too (unless I'm thinking of another place, which is entirely possible) Water clarity might be a factor too, but it seems that So. Texas' water, at least south of POC is about like the basins in the Keys....at least this is true for the Laguna Madre.
I'm not sure what sound does, at least in deeper water, 30 ft. or more. First, tarpon hold under the bridges in the Keys, where car and truck traffic (noise) is obvious. Perhaps worse would be the vibrations sent through the water when one of those big semis passes by. I have felt this when diving for lobsters around the bridge abuttments down there, so you know the fish feel it. Yet folks catch tarpon from under the bridges all the time.
Down here in open water with no boats around, one might think that engine noise would have a different effect, but it doesn't, as long as the operator doesn't change the rpms of his engine when the fish are near. This way, the fish become accustomed to the noise as it approaches...or as the fish approach the boat, as might be the case here in So. Texas, since the fish here are mostly all migrating and moving pretty fast. I can see where a school of tarpon would overtake a boat that was slow trolling in the same direction as they were moving.
One time in the Keys, I was running the bakcountry early one glassy morning. Off in the distance I saw the glisten of tails in the sunlight, about 500 yards away. I could not slow down because the water was too shallow. They were on a flat at the edge of a small channel that I would be turning into, right where these fish were. So keeping my rpms the same, I ran my boat to within 20 feet of these fish, which turned out to be 4 bonefish. They didn't spook until I had to turn into the channel and reduce my speed to make the rather sharp turn. So I don't think that tarpon in deep water are going to spook.....again, as long as you are idling and do not change your rpms.
This is just conjecture.....I have never trolled for tarpon. I just know that people do it successfully in the Keys and also in So. Texas, at least according to the videos I have seen right here on this forum.
PS.....we also used to drift and jig for tarpon in Key West harbor. I'm pretty sure we kept out engines running when doing so. Again, that water is 32 ft. deep and the fish were eating very close to the bottom.