There are a plethora of options. iPhone, iPad, Laptop etc. all with varying abilities, requirements, and costs.
In the iPhone range... iNavx, iSailor, iBoating etc. All have features and functions that you will see similar to your Garmin unit.
Many offer free access for a limited time or with some of the features not available. Not all this is dependent on you knowing exactly where you are. Cell Phones are not like GPS systems in locating you. So since you can not control (trust) the cell phone location you will want to give "EXTRA ROOM" from the navigational hazards as you find your way in the fog.
As you say they are better as a WTSHTF situations and are better than having nothing. You might also consider paper charts and learning to navigate using your compass and depth sounder.