People I know who are a lot more knowledgable than I about sonar feel that Garmin makes a better unit than either Lowrance (also Eagle) and Humminbird. I have a Lowrance 350 that has been back to the factory about three times because of the seal. It's still not sealed, and this one is going back again. I'm going to sell it once repaired and replace it with a Bottomline that has GPS and mapping. I don't have a lot of recent experience with Bottomline, but their reputation is fairly good.

Wattage and power are important to give both target acquisition and separation. For fishing in the salt, you want a transducer with a narrow beam (about 8 degrees or so)-- in order to pick fish in the vicinity of your boat. Wider beams pick up a lot of stuff that is not even close when you're shooting in deep water. If you realize that and use the info to help you pick a general area to fish (rather than just a spot or drift) that's fine. but if you're trying for more specific info, you won't get it with the wide transducer.

On another note, wide beams allow you to see your downrigger ball and adjust its depth according to the level of fish activity. Also, wider beams give you a better idea of what's near you when fishing in shallow water. Like everything in life, transducer beam width is a trade-off.