As a 'droid user I do have to ask - what use is a plotter that the battery will die in 20 mins?
Seriously tho' - I planned to use my 'droid phone with the MemoryMap app as an emergency plotter - but very quickly found all the things talked about above very true - even the condensation in the "dry" bag confused the touchscreen... I brought my old "pushbutton" Nokia out of retirement so I could at least phone people!
I have reverted to my 20 YO (!) Garmin 12 as the backup GPS - which also has the advantage of:
1) if it's a boat power failure, it will go 23 hrs on a set of AAs
2) IPx7 rated
3) chucks NMEA out the back so keeps the DSC "fed" too (assuming it's the main plotter has failed & not the boat battery)
Bottom line is you can buy a wee handheld mapping Garmin or equivalent for about £50-70 or so off ebay, which does what it says on the tin.