2d shows "cartoon images" of stuff under the boat...

DI shows "picture-like images" of stuff under the boat and out to the sides a few feet....

SI shows "picture-like images" of stuff under the boat, out to the side +100 ft, and which side of the boat...

Decide what part of "under-your-boat" you expect your electronics to show you then buy the technology that best accomplishes that desire....2d...DI ....SI....

Rickie