For several years, I have logged soundings for a lake that was low by 3 ft. This became normal pool level. Now, the lake level is 3 ft. higher. This is now normal pool level. As a result, I edit my new soundings by -3 ft. Can I edit the map and increase the depth by 3' so I don't have to constantly edit my soundings?

Yes, you can apply an overall water level offset to the entire map, starting in RMv2. If you have adjusted all of your tracks to a particular pool level, then the best thing is probably to stick to this level for all of your new tracks as well, and just adjust the map water level offset to the new level.