A pleasant surprise, with more new features than I expected.
Painting biomes in your exhibits has been nicely improved: previously, one would select a terrain type for any given biome - anything between deep water to a "full-featured" terrain that would include trees, shrubs, and rocks. One could not, for instance, paint using the shore terrain type and have trees painted along with it, or paint using the far-right terrain type and NOT have any trees/shrubs/rocks painted. This has changed - we can now paint in any terrain type within a biome, and manually select whether we want trees, shrubs and/or rocks painted at the same time. This allows for much more freedom while painting terrain in an exhibit, as you can easily paint a wooden area with no rocks, or a rocky area with no shrubs, etc. Very nice.
Another welcome addition is the eyedropper feature: just about any item or animal in the game can be cloned from an existing item/animal in your zoo (exceptions are items/animals that have not yet been researched or are not yet available in your zoo.) Just click the existing item/animal you want to add more of, click the eyedropper button in the open panel and place it in your zoo. This makes it really convenient to add more of an existing item/animal, without having to re-open the related menu.
One of the things that I found most impressive and fun was the manual training of animals. Marine Mania indeed adds marine shows to the game, but animals will not perform unless they've been trained to do particular tricks first (some tricks are locked until your animal reaches a certain skill level.) There are also several skill levels for each trick, which can increase the popularity for that trick. Note that animals can easily be trained by the new trainer staff member. The trainer can be set to train any particular tricks you choose to any animals you specify, or simply train any trainable animals in your zoo if you don't specify anything - very handy.
However, if you want to have extra fun, you can train an animal yourself by placing a training arena in their tank. This brings you to a close-up view in which you're asked to manually train the chosen animal by performing hand gestures with your mouse. Essentially, you must follow a "tracker" that moves on your screen in a particular shape, and click your mouse button or press certain keyboard keys at specified places (it gets more difficult as the animal's skill improves.) If you fall behind the tracker, or don't follow the path well enough, or don't click/press where indicated, the training for that trick will not reach 100%, or may even fail.
Once you have some animals trained, you can create and schedule different shows, mixing different animals doing tricks together or separately, or edit existing shows to add or change the tricked performed. There are truly a lot of options for this, although it is not too overwhelming.
Two new tabs have also been added to the Zoo Quick Stats panel. We can now view more details about individual buildings (restaurants, shops, etc) in a zoo, as well as individual marine shows statistics.
There are many useful options for building tanks or hybrid exhibits (exhibits that include both a tank and land, for animals such as otter, sea lions, etc.) One can raise or lower all tank walls or just a single wall, raise or lower its floor to increase/decrease a tank's depth, and more.
Another reviewer mentioned poor sound in this expansion. I didn't notice anything different from previous expansions. I was actually impressed with the realiastic underwater sounds when swimming with animals. I personally think the graphics and sounds are very adequate for this game.
Pros: new useful features, lots of options to create marine shows, nice choice of marine animals.
Cons: a few minor bugs related to terrain editing (I had problems flattening small terrain areas for instance -- they would just not flatten for some reason.)