According to A Scientific Case for the Mariana Trench Marine National Monument there are at least 19 species of marine mammals in the waters surrounding the Mariana Islands (I've including a link to a pdf version of the report. Let me know if you want a hard copy). We found two of them.
We saw spinner dolphins off of Uracas and Agrigan and we saw these guys off the coast of Alamagan.
There were probably more marine mammals near our boat, but we didn't have listening equipment and we weren't really looking for them.