2d shows structure under the boat
DI will show a picture like image of the structure under the boat and out to the sides a few feet...
SI will show a picture like image of the structure under the boat, out to sides 100+ feet, and which side of the boat

In 1 pass SI will effectively show you a 200 ft wide swath of water...eliminating unproductive water...

SI is invaluable for finding new unknown structure...

Rickie