The "All Aboard" time is 3:45pm. Everyone should be on board by then. The only folks that won't be are those that are coming in on cruise day and their flights have been delayed.
DCL will "hold the boat" for those folks that purchased air with their cruise.
As far as doing something at WDW on embarkation day, you can. It's a short day. If you are at EPCOT or MGM you can have a little more time in the park because the parking lot is fairly convenient (never have parked at Animal Kingdom - it may be the same). If you choose to spend the morning in the Magic Kingdom, you will have less time because of the monorail or ferry to the parking lot and then getting to your vehicle in that huge parking lot. Regardless, I would want to be in your car by 1pm at the latest. It will take you a good 1hr 15min to get to the port from WDW.
For what it's worth, on our first cruise we were in the parks until noon, grabbed a quick lunch then jetted over to the port (arriving around 2:30). We made it in plenty of time, but we never felt like we got settled in until sometime on the second day (and we were on a 4 nighter).
We now arrive at the port around 11am, get checked in, get in line, are in the first couple of hundred people on board, toss our carryon luggage (may not be able to do this now) in the room, make Palo brunch ressies, check the kid into the club, and then enjoy our lunch. We then wander around the ship or go swimming until just before the safety drill. We start unpacking and getting organized in the 1/2 hour before the safety drill and for a while afterwards. We then head up to the sail away party and we are ready for the next week of fun onboard and on shore!!