Chrisso,
It is easy. Buy the plastic coated Qld Transport chart of Moreton Bay. It is gridded up for Long & Lat. Use a superfine tip permanent marker pen & put in your marks.Identify your marks numerically as per your GPS or renumber them. They won't be in their exact position on the chart this way but your GPS will get you to them.
When you head out to fish a particular spot, refer to the chart & bring up the GPS numbers that apply. Low tech but it works.