Yea, if you already have, or might possibly have any investment in the Google Play app world, you should try, as much as possible to start with apps that work on both platforms.
If you are using it primary on the boat, consider reader apps with offline capabilities (i.e. Amazon, etc...).
Dropbox and other cloud storage solutions, including iCloud, require internet connectivity to use them, so don't forget to keep offline/local copies of important files you may need.
I'm personally not a big apple product user, but like you, I have a somewhat proprietary use for an iPhone5S. The form factor and some of the apps available make it an ideal cycle computer. All of the capabilities of a dedicated Garmin unit, plus the added benefit of being a small, pocket friendly smartphone.
One last tip, you might want to consider an external battery and charging cables, and possibly a solar charger for it.
I've had good luck with Anker's products for this.
http://www.ianker.com/