If it is a known image type like png, jpg, tif etc you can use Weatherfax_pi to overlay it with transparency, but first you must import
it as a file and edit it with the Image wizard to set two coordinates (lat,long) and the type of map projection, etc. You will need to make a new coordinate setting (it is a dropdown selection) for this bathscape and remember it.
Once you have completed that the Image Wizard will complete and overlay the bathscape on the charts
. You can adjust the transparency. You can edit it again iterative style until it is right.
Then so you can bring it up again easily, you'll need to save it out as a KAP file somewhere and treat it like a chart. There are some tricks to use it such as setting up a chart group for the bathscape and pointing it to the directory where you have the new kap file.