My smart phone uses a simpler Navigator program not as good as my TomTom Navigator. It is online but niether unit is good enough for use on the local lakes. I also have a >agelan GPS which can be used on lakes, rivers and unmarked roads. I recomend you check with a shop that specialises with outdoors activities to see what is available and fits your needs. Save the phone app, they are ok but just not accurate for lake use.
Many smartphones use something called aGPS, or assisted GPS. They use the cellular network and the GPS radio together. If you are away from land, and a cell signal, the GPS my not work well, if at all.
From the Apple site: " iPhone pinpoints your location quickly and accurately using a combination of GPS, Wi-Fi, and cellular towers."
A buddy really recomends an app called Navigon. It isn't cheap, but he uses it on land and water. He isn't crossing the Atlantic - just fishing in local lakes.
My Magellan hold two penlight batteries and works 4-6 hours on them. It will do topagraphical information , which I haven't seen on my phone. I have wo wonder though with all the comparrisons this app does, it wold go dead earlier if it weren't plugged in to another power sorce. I also have an early modle fish locator by Eagle. I don't want you thinking that I think I have all the answers because I rarely cath anything, but a cold. Still, my choice is a GPS. Just because it leaves my phone to be there for an emergany.
I have been using GPSs since before they were available to the public. For quick routing, etc I would use my iphone, however if I were out in the woods, I wouldn't rely on it. I typically have my Garmin Oregon and as a back up my Garmin 60CSx. And never go anywhere out of the vehicle without the tried and true, map and compass. Technology can fail.
That said, I have found that the gps accuracy on my iphone is excellent. I'm not sure about out on the water, but I have used it geocaching where there is no cell signal and relying only on the GPS portion itself. The mapping programs are not always the best so I would research them first. Also keep in mind that most of the programs do not have on-board maps and rely on a wireless signal to download the maps as you move along.
My smartphone is the G1 by Google for T-Mobile and at first there were only a few for people to get and try out. I too would like to check out the choices out there today. My G1 has limited memory so if I load it up iit will slow down. What all of you mentioned sounds good.