From looking at the Boathouse menu on allears.net, there is a Surf & Turf special that includes a 28oz Tomahawk rib chop and lobster, serves two people for $115. From the Disney Food Blog, it says that a meal that serves 2 might require TS credits for two people, which for a Signature restaurant would mean 4 credits.
Since you also couldn't find the steak and lobster entree on the menu at Narcoosee's, you might want to call Disney to see if they can help you find the meal you want.