Hiya
Coming from a commercial fishing background, having and being able to use a decent echo/fish-finder is extremely important. I've used many makes, ranging from entry level units to top of the range units from manufacturers such as Garmin, Lowrance, Eagle, Hummingbird, JRC, Koden, Furuno, Nav-man, etc
As mentioned before, placement of the transducer is critical for the best performance on a unit. For spearfishing purposes, where a spearo is mainly looking for reef and structure, not really schools of fish, any entry level unit will work well. How-ever, once you start needing to find pelagics and schools of fish, a colour unit is definitely a better choice.
I've used the Garmin 120 (pre-decessor of the 140) and wasn't very impressed with it. Similar to their entry level unit, so not really worth the extra money.
I can HIGHLY recommend ANYTHING from the FURUNO make. Their LS4100 is a fairly in-expensive entry level unit, but works BRILLIANTLY. It shows squid on the bottom and fish, flat on the bottom, EXTREMELY well for a monochrome unit!!
Hope that helps!!
Regards
miles