I'd say it was a white shark almost for sure. This is the time of year they are very common, as are attacks. I saw one attacking a sealion a few months back. Its hard to believe what you are seeing while its happening, but when you replay it in your head usually there's not much doubt. I can say for sure it wasn't a sevengill, soupfin, thresher, or hammer. Sounds like you have lots of hours on the water... you would've easily identified them by their behavior and appearance. Possible to mistake a mako for a white though....
Also as an aside, we do not have porpoise here. We have short-beaked common dolphin, which are often incorrectly referred to as porpoise by fishermen. Pointless detail, I know, but while we are identifying animals we might as well be consistent. Don't even get me started on the seal vs sea lion thing
