We recently came back from the BVI and I brought a Garmin hand held, GPS MAP 378, an iPhone 6 Plus and an iPad Air both running Navionics, iSailor and Garmin Blue Chart Mobile..
Here's how it went down in the tropics..
Garmin - Screen bright & visible even in direct sun and 100% reliable
iPhone 6 Plus - Screen very tough to see even on max brightness. Running Nav programs munched the battery. Had 4 over temp shut downs in 7 days. Horribly unreliable during a short 9 day cruise.
iPad Air - Same as above but 6 over heating events leading to shut down in 9 days. Horribly unreliable on a short 9 day cruise..
Even when I took the devices out of their
cases they still over heated and became rather slippery. I have also had my iPad over heat here in Maine as well as freeze up when it thought it needed to update something.... This is often a full 20-30 minute shut down and cool off before rebooting.
I consider my iToys just that, toys.....
P.S. The Navionics charts were quite poor for the BVI, IMHO. The Garmin charts were better and more accurate. I always had them running side by side when the iToys were not in over-heat mode....