I would probably take the boat to MK, and the bus from there to POR. The boat is not speedy, but it's a very pleasant ride, and should be pretty empty at lunch time.
The bus from MK to POR is pretty quick, maybe 10 minutes (but it may circle the resort before arriving at the South Depot, which is closest to the Sassagoula Float Works.)
On the way back, you might want to take the boat from POR to DTD (which is an even nicer ride, IMHO), and transfer there.
For reference, buses to DTD from the resorts start at about 8:30 a.m. Full service from DTD to the resorts starts about 11 a.m.; before then, buses run to the nearby resorts, and are dispatched to other resorts as needed (which can take a while.)