The GPS I use at the helm is an old Garmin GPS 12. It has all the info I need right there such as speed, course, distance traveled, and elapsed time. Since my GPS is connected to my autopilot, I can enter a waypoint and let the GPS steer the boat. Or, I just steer it myself. The GPS has no maps or charts. I have those down below.
Now, if you want to use a smart phone or tablet, that will also work. There are lots of applications that use the GPS receiver in the phone/tab that do not need cell access. They are stand alone. However, a lot of these devices are not really good in direct sunlight and do not take kindly to excessive moisture.
I have an Android and use GPS Test app that gives me just about all the info the Garmin 12 gives me. I also have a few charting apps that also give me visible location. Multiple backups on the boat. However, a simple older GPS will work just fine.
You can also look on eBay for older GPS terminals. That is where I got mine.
Good luck!