Perhaps your iPhone might be a good back up for a marine gps, but I'd be very hesitant to put all my eggs in one, non waterproof basket. As you said, keeping it in the cabin is a possibility (very limiting should the weather require seeing the gps at the helm), but a cup of coffee or a soda could leave one with no gps at all. For around a hundred bucks, it makes no sense not to have the right tool for the job.
The op said he would use it for coordinates, then plot using paper charts. Most do this type of work in the cabin, not at the helm.
I agree that the boat's main GPS needs to be robust and intended for marine use. However, if the user is merely looking for coordinates to practice analog navigation near shore, an existing mobile device is a good and free option. He may subsequently decide he needs a real chartplotter not just a gps, and he's saved his $127.
Also, you can get cases for the phone that are waterproof and very tough. And the cost of the case might be justified in the diverse use of the device. IE it's good to have one anyway.
Everyone's situation can be unique. For the op I think he will do well to practice with the free option then decide what he ultimately needs.
I have a handheld. The Gpsmap 76csx. Fabulous. It's at the helm so the small screen is not bad. I put it on full map with no data boxes, and use the AP control head for data boxes. This allows the little screen on the garmin to be reasonably sized.
The main reasons I like the handheld are that it uses a tiny amount of power, it's easy to remove and stow, and it has battery backup - if there's an electrical issue with the boat, I would still have many hours of navigation ability, used intermittently for days. Additional AA batts aboard allow long term use on the event of electrical failure.
I also use an iPhone as backup, and a PC tablet as 2nd backup and planner, and for loading free charts of far-off islands that I can dream of visiting.