We always combine a WDW stay before or after our
DCL cruises out of Port Canaveral. We always book separately. There really is no benefit to book your WDW stay thorough DCL anymore. As mentioned you can take advantage of any offered promos WDW is offering if booked separately. You no longer have to stay at WDW resort dependent on your cabin type on DCL. You have the flexibility of staying at a value resort if you want and a verandah on the ship or any combo of accommodations. You also don't have to limit yourself to just 3 or 4 days at WDW or 3 or 4 days on the ship like you use to on the official land/sea package. You can stay just the night before at WDW if you choose or a week. It doesn't matter as each reservation is separate.
You can easily make it a seamless vacation from WDW to ship if you purchase the DCL transfers that will pick you up at your WDW resort and bring you to the ship. The cost of this transfer service is $35 pp each way.
You can also rent a car for the day or get a towncar service. You would have to compare the prices of the different modes of transportation.
MJ