I guess I could go with the +8, but the problem is that this is supposed to be a regular 1 HD familiar (although I don't get why it's a magical beast other than the DMG says so). I don't want it to be so much more powerful than other familiars that it's almost godly by comparison, but I don't want them to lose out on everything too.
I wouldn't consider the Poison Use ability simply because vipers and other familiars with poison don't have it. They don't apply poison, it just gets excreted from the tips of the spurs. It's like a regular snake. They're not immune to their own poison, and they can accidentally bite themself. A platypus is no different it would seem. I did read that only the males only have poison claws, but the magazine says that's what you're to replace it with, so I'm going with it.
I'd probably drop the blindsense down to 20 feet. It's just that the dire version had 20 feet blindsight and 60 feet blindsense, so I just halved it. But yeah, it's circumstantial under the water anyways. From what I read, the water makes it easier to conduct the ability, much like electricity. At least that's how it sounded.