( i don't know if this is the right answer but..... i'll try)
i think it is the dolphins... like there are 129 species of marine mammals i don't think the taxonomists and scientists cared and studied for the first mammal in the water... but i still think its the dolphins...
There are no specific answers about this because for some reasons, they can't predict who really came first, but my answer would be---- it is either dolphins, whales or sharks.
