Here’s the nerd coming out in me, but after missing out on a deep dive because someone had the wrong marks I’m now taking control of the situation. 
I was going to buy a GPS, but after some research I found out that Navionics make an IPAD App that is basically a full blown chart plotter and GPS for $45. It includes Australian and New Zealand waters, and is very detailed in terms of chart plotter information. I’ve only used the app out in the wild once thus far, but it paid for itself and certainly helped us located and dive a new wreck.
The only issue I’ve found with the app is that it doesn’t bulk import marks from other sources, so you have to enter this manually but it’s not the end of the world. You can export anything that you enter or snapshot though.