The port area opens at 10, the counters for check in open at 10:30. If you show up before 10, you will not be allowed on port property. By arriving any time before 11, you will be one of the first to board (but not THE first), and have plenty of time to make reservations and do the "urgent" business on board. Remember that your party can split up to do ressies. DH can go to guest services to get Tea with Wendy tickets while DW goes to make Palo ressies or whatever.
We try to arrive between 10:30 and 11. My teen gets in the boarding line while I check in, then I join her in the line.
The busses from WDW do not even leave WDW till noon, and people from the airport arrive all day.