My intuition, based on what was written on the extraordinary challenge in writing the app for Macs, that we should not hold our breath. The market share is probably not worth the effort, but for the cost of the screensaver (2) I wish it would have been supported for more than two years.
Jim stated above that Marine Aquarium 3 is in development for Mac OS X Snow Leopard. Considering the improved graphics (MA2 was built for 1024 x 768, MA3 was built for 5000 x 1600), I think the reasonable upgrade cost will be worth it.
Also, it was Apple's decision to change their APIs and break screen savers that are less than 2 years old.