If the "Lettering" you refer to is the left side readouts, you can turn them off in the Sonar Menu item SI READOUTS
That only works in the full screen SI view.
To eliminate the SI water column and the double center line, go to the SI express menu (push the Menu button once with the SI view displayed), down to SI Enchance--choose that, then down to Contour Mode. Set that to ON
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1