I am Filipino-American (Tagalog, to be specific), and after I watched Disney’s Moana, it sparked a longing to find what was left of my people’s culture. Unfortunately there’s not much to find, and most of it was written down by the Spaniards, who called us "ridiculous" and "backwards" before they converted us to their people’s God. There are several wonderful Fil-American authors, but they are also limited to the things that Spain couldn’t destroy, and the Christianized remains of mythology and traditions.
I can only find one sentence about Haik, in various forms, and that sentence was written by the Spaniards. "Haik is the Tagalog sea-god, whom the natives invoke before all sea-voyages."
The Crocodile God is not meant to be historically accurate and painstakingly researched--I have tried researching, but as a Fil-American with limited Tagalog skills, there is only so much I can find.
So that would mean at least 90% of this story is a mythopoeia, where real Filipino folklore is patched together by the traditions shared with Polynesians/Pacific-Islanders.
Of the "real" mythology in this novel: We have a strong tattooing tradition, called tatak in Old Tagalog while the modern revival is batok; tattoos were statements of community ties, personality, prestige, and extensions of clothing, and receiving one was a sacred ritual (such as for coming-of-age and important events like marriage). Our pre-colonial ships and boats can be swapped for Polynesian boats without a lot of trouble, and truly precolonial ships have triangular crab-claw sails, instead of the square European rigs. Bathala and Lumawig are very obvious Filipino counterparts to the Polynesian Maui. Many of the gods in this story are real deities with artistic embellishments, aside from Haik’s lost whale-goddess daughter who is very much fictional.
We revered crocodiles and called them dragons; our sailors, warriors, and fishermen would get crocodile tattoos for protection and intimidation, and we put crocodile-heads on our ships and swords for the same effect. Pinoy mermaids have a pretty spotty record with the islanders; some tribes believed they were beautiful and friendly, but others thought they were man-eaters.
Baliti trees are said to be the homes of fairies and ghosts, and a lot of older Filipinos refuse to touch them or get too close for fear of getting cursed. Filipino shamans had a whole range of names (the most common modern term is the Visayan’s babaylan) were often women, and people who would be deemed part of the LGBT spectrum.
Everything else? Your guess is as good as mine.