You definitely can book onboard but the pricing is the best early booking savings available to anyone, no discount. You DO get the Castaway Club credit, though, and the only way to get that is to book onboard.
Any onboard booking can be transferred to a TA (such as Dreams) for any additional rebates or discounts offered.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!