John,
The track-point filter is a destructive filtering operation that permanently removes track-points outside of the specified range. This can be undone, but only whilst the project is still open. Sorry to say that if you have since closed this project (or ReefMaster) then you will not be able to restore these track-points.
There are probably better ways to achieve what you want in this situation - e.g. adjust the overall water level by the required amount (a new option in the map projects toolbar) and then either removing or recolouring the isobaths that appear above your new waterline.