Which is best for marine apps? Apple's iPhone and iPad, or Android? There are pros and cons on both sides.
First, it must be said that most boat owners use an iPhone or iPad when off the boat, so their natural tendency is to stick to the same platform when on board. But is it worth changing, with all the hassle of learning how a different device works? There are a greater variety of apps written for Apple devices, and if you want to run a specific app then it is probably available in the iPhone store. The big exception to this is OpenCPN, which is oly available on Android as far as mobile devices go.
A big benefit of Android devices is that they are multi-tasking. On an iPhone or iPad, if you are running your nav app and then want to bring up a different app, the operating system will force the nav app to close down, and it will disconnect the connection to your instruments, so when you bring the app back up, it will have lost a chunk of data. Also, if the app isn't that well written, it may not properly close off the link to the instruments, so your NMEA to Wi-Fi device may be left with an open connection, and when brought up the app may think it is still connected to the instruments. Of course, if the app is well written then the connection issues will be properly handled, but you will still have lost a chunk of data. On an Android device, the app will continue logging data in the background, and the data connection will not be dropped (again, if the app is properly written).
Another advantage of Android devices is that they are cheaper than Apple. Most boats are at best a damp environment, at worst very wet, and very few phones or tablets are designed to be water resistant or waterproof. So sooner or later it is likely that your phone or tablet will die, either from water getting in, or in the longer term from corrosion due to the damp, salty atmosphere. With Android devices being that much cheaper, it is a less painful experience when they fail!
So, in conclusion, the first thing to do is to look at the apps you want to run, and see which platforms they support. This may make the decision for you. Then, if you have the choice, would recommend having an Android tablet for the boat.