So after discussing it with my wife (who, it turns out, has always wanted to stay on a houseboat!), I booked it this morning.
We just let the kids know about it, and their reaction was priceless.
Me: We haven't been on vacation since May and weren't planning another one until December, but Mommy has been getting a bit antsy so we've been looking into different vacations. So this morning, Daddy booked a timeshare in Oklahoma.
Kids: (Nothing but stares, with a "have you lost your mind" look on their faces.
Mommy: On a houseboat!
Kids: (Total change of attitude, with excited cheers and lots of questions!)
I do agree that the $900 appears to be a ripoff, unless owners get charged the same amount, but the total cost is unbeatable. $900 fee + $209 exchange fee + $50 (5 TPU @ $10/TPU) = $1159 for a week. If you go directly to them, they rent the boat for over $2000/week, and from my limited experience renting other boats (often hundreds of dollars just for half a day, for a much smaller boat), that's a good deal.
We're certainly excited about it!