Fish Finder, which one please ?

Hi Folks
I am not a fishing guru, but well into Electronics.
A close friend who have 50 ft fishing boat, operating in Asia Pacific
waters, targeting 5-25 pound fish.
He asked for my advice on suitable, yet less costly FISH FINDER.
where else to ask but this group, plz advise me, and God bless you.
please note that, the owner of the boat contributes some of his
profits to the needy of those islands...so by helping him, you are also
contributing, and I ask God to bless you regardless of Ur religeon or
belief.