For Saltwater the HDS 5 is pretty hard to beat. Compact, good resolution, Downscan, and HD. The comparable models would be the Garmin 546s, or maybe the Raymarine a50d. I have used all of these models on boats and I have to admit that the Garmin has the best waterproof and map abilities with the G2 vision card. But even though the maps are not quite a pretty the HDS 5 is geared more towards a serious sonar for the serious fisherman. The cheapest cost on any of these Combo units is $799+! So be sure this is where you really want to dump your hard earned moolah!
P.S. My Valentine's Day gift from my wife was a new HDS -5... A great way for her to dump her hard earned Moolah!

It has not improved my fishing any but... if I can just get it to find Lobsters!