I also hav an elite 4 and I can tell u they are awesome for estuary but it is true deep water they cannot read structure properly and fail to read fish. On my sounder once I hit 15-20 m the bottom is just flat very very flat but from 0 -13 meters I can see every piece of rock/ weed like its 3d
Anything over 20 m is no good for the dsi.
Most important thing is what is your budget?
Seeing how your fishing deep water you would need a decent sounder with a frequency of 83mhz