Not too sure about the need to fish sea mounts for big dogs at Marion. On the two times I've been there the biggest dogs where all in less than 70m, either outside the passes or around the lagoon bommies. Chances of being spooled, almost nil, chances of pulling the fish away from the bommies and sharks, well, just roll the dice and let the fun begin!!
By the way the biggest we saw stunned us all, an educated guess by some seasoned observors was +120kg. Good luck.