Fish Finder Help

[MallenManson]

I've always been a shore fisherman but finally decided to register my tiny little boat. Bought the trolling motor and got a fish finder off ebay, today in fact.
when i checked it out, I see that it's single beam and it projects 20 degrees.
from what I've been able to gather, 20 degrees amounts to 1/3 of the water depth. I'm fishing inland lakes in Michigan and will be spending most of my time in 10-20' depths.
So at 10', I'll be monitoring fish in only a 3'4" path right?
If i get a dual beam, supposedly i'll view a 60 degree path, so 10' path for 10' depth?
This is my first finder and I went into the purchase totally blind.
Humminbird Piranhamax 210 is what I bought.
Any help would be greatly appreciated