depth not giving accurate read...

[rb56]

i've noticed several times in places i know the water is deep but my dvs4 will read 3.1 ft. or there about and as low as 2.9 ft. yet i know the water is about 20 ft. deep. any ideas on what can cause that? it usually gives that read for some time...maybe 20 minutes at times. also i notice that the feed that comes into view from the right side of the screen will freeze and may not begin to move again for maybe 1/2 mile...suggestions on these two errors? i do love the screenshots i get from it and the visual on screen with the copper vision is unreal.
i'm thinking the first image is a very large school of baitfish but not sure. the second image i believe is brush with fish in and around it. i believe those are large rocks on the bottom as the lake is full of them.