From what I looked up, it, there hasn't been enough study into possible dialects of Hawaiian although from the small bits I could find it seems there's two possible dialects: the Ni‘ihau dialect (spoken solely by Hawaiians living on the island of Ni‘ihau) and the O‘ahu dialect (spoken pretty much everywhere else on Hawai‘i. So most likely we're learning the O‘ahu dialect which is probably considered to be the main dialect of Hawaiian at this point.
(I could be wrong about the dialects, what little research I did about Hawaiian dialects didn't turn up anything super concrete.)
There was no standard Hawaiian form prior to the arrival of outsiders. Once missionaries settled in the islands, there developed a need to do instruction in the language and to translate the Bible. Thus, it was necessary to standardize the language. It favors O'ahu and Maui, but In reality, Hawaiian differed slightly from island to island, district to district, and even family to family way back when. It would be impossible to say it is based on just one dialect. That said, after more than 150 years, the standardized version that is taught to others is like its own dialect and is called informally "the university dialect" by some. Once you learn one way of speaking to the point of being consistent, you can learn the variations that make up the spectrum found throughout the islands.