I'm kinda new to the iPhone, but it has a Compass application included.
It would give you long/lat and direction.
With a map/chart, you should be able to navigate just fine.
There is also a Maps application.
It seems to need a cellular data connection to work properly.
So, if you were on a lake, river, or any sort of inland waterway, you'd be good.
Normally, it gives a street view, but you can turn on a satellite-view overlay.
I'm sure it can pull up points of interest, fueling, restaurants, etc.
This barely scratches the surface.
A dedicated ocean-going application could do far more on a smart phone.
But if it were my only tool, I'd darned well be sure the phone was kept in a couple of plastic bags, and out of the salt spray.
Water damage is easy to suffer on a delicate piece of electronics.
I inspected the handset of one friend, and noted the water sensor in the bottom showed signs of moisture.
She said it was from being in a steamy shower.
