I would most DEFINITELY recommend that you get a chart plotter, if the money's available.
A normal gps works very well, but the chart plotter is VASTLY superior when it comes to safety at sea.
With a normal GPS, one has to make up routes to and from your fishing spots, so as to avoid any navigational dangers, like rocks, reefs or shallow area's. With a plotter, there is no need for a route to be made, as you can see exactly where you are in relation to your destination.
So, whats the difference then?? Well, assume its night time or raining or heavy misty/foggy conditions, where vizibility is severely compromised. You are returning from your fishing spot, running for home, on your GPS's track/route that you have pre-programmed. Now, should anything block that route, ie. a large ship blasting its fog horn and you don't have a radar, or another boat reports hazardous floating debris right on your route, how will you be able to SAFELY navigate back to port?
Another scenario. You launch from your usual harbour/port/beach and go fishing. The sea picks up and you're forced to return to a DIFFERENT harbour or port. Chances are very good that you will not have a safe route marked on your GPS. Even if you have engine failure and need to limp to safer inshore waters, a plotter showing you ALL the reefs and naviagtional dangers will be invaluable.
I always urge everyone to invest in a chart plotter!! Not only is it a safety feature, but it also is a excellent fishing tool. It shows your the contour lines, shallow reefs and much more info. Its like having a huge paper navigational chart, with all that information, open right in front of you all the time.