I agree. You are not the only person to say this, but first of all, technical issues need to be resolved.

we don't have animals diving yet. Therefore, having too much expectation is disappointing. Treat it as just a wish, don't get too carried away. Instead of expecting too much and being disappointed, don't expect too much, but if you get a DLC like you mentioned, you'll be twice as happy.

If the diving system of animals is added to the game, I think that the more demanded marine animals will come first, for example penguins, otters. I'm not saying I don't want a big DLC, who wouldn't want that! just, I am assuming we will get a few marine animals with DLCs that will be released earlier.