I have the helix 5 on my aluminum boat. No issues with the image quality etc. Same as my 898 minus the preset buttons. However, I have concerns about durability. My helix 5 crashed about 3 months after purchase. Would go off every time I hit a bump and would not get GPS fix at all. The unit was fixed under warranty.Was on the lake last weekend and it started to do the same thing again. It's a few months out of warranty now. I've always had great luck with humminbird and I'm not trying to trash their product at all, but you should get more than a year and two months out of a unit. Mine is probably just a lemon but I will not take another chance on a helix unit at this point.
Do you have the cable collector fully seated and have adequate power? That includes clean and tight connections to the power source, and a healthy battery that can perform all the functions connected to it.
If you didn't connect the power cord directly to the battery, try that.
Failing GPS is a sign of inadequate power.