There is apparently one main catchall category for Sea Monsters in the Philippines generally, and it is the exact equivalent to "Taniwha" and literally means "SHARK." It basically means the Whale-Eater but in this case ,the Long-Necked Sea Serpent is depicted but using the same name as used by the whale-eater (the creature serves mythologically both as the Lord of primordial Chaos and as the snake that swallows the sun during a solar eclipse. I imagine the last myth is the original for Jonah and the "Whale." The composite SeaMonster equivalent to Taniwhas is presumably the same as Tompandrano also and there might also be a linguistic connection. In this case, though, we are talking about a standard "Periscope" sighting such as elsewhere seen the whole world over.
The creature has an unusual-looking thing in its mouth which to me resembles a large crab. As far as I am concerned it actually IS a large crab the creature is attempting to eat. The best candidate would be the blue swimming crab or flower crab as it is known in the orient, and this can be something like eigh inches across the carapace: http://en.wikipedia.org/wiki/Portunus_pelagicus
The scale is shown as if the Longneck was about 60 feet long with a head two feet wide and three feet long. The head is shown in a correct shape and proportion for a Longneck, my personal preference is that the size represents a theoretical maximum and not a commonly achieved size. The representation is similar to what is called a Naga further to the West of the Philippines. I am assuming the illustration is based on older traditional prototypes. The size and the shape of the creatures head (Plus the confirmation that Longnecks are observed to feed on squids, octopi, lobsters and crabs as well as fishes) are statements which can be found in the conclusions section of Oudemans' book The Great Sea Serpent.