Looks like a cable (or anchor rope...??)...going from the surface and attached to something laying on the bottom...actually 2 cables as they show as shadows in the first SI pic...and barely discernable in the dark water column...
I see similar cables in my home lake marinas where the floating docks are cabled to huge concrete anchors..
You might experiment with bringing that SI Range in to make the targets
bigger in the SI image...your SI images actually have really good detail...but the longer SI Range makes the targets pretty small...
I very seldom run SI Range past 65-70ft for waters up to 20ft deep...or so...
Rickie