First thing I'd do is get a multi meter from Harbor Freight. An acceptable digital is less than $7. Put it on the power terminals on the plug that goes to the depth finder and see what the voltage reads with the main in off, start and run. Off 12.6, Start 11.6, run 14 would be expected.

Some finders are oversensitive to battery voltage and do a shut down when no one else cares. I have a Fusion radio and an old Lowrance HDS that occasionally do the same thing. Never when starting my T8 though.

Lowrance HDS Gen 2 Touch spec is 10.8 to 17 vdc. which should allow it to continue running during a motor starting draw down. I personally do not trust the 10.8 bit.