1. Going out Intl. Gateway to the Boardwalk resort, catching a bus to DS?
First of all, if you plan to leave via the International Gateway to catch a bus, walk over to the Beach Club to catch the bus. It is much closer to the Gateway than is the Boardwalk. Starting the clock as you leave the IG (as opposed to, say, Ellen's Energy Adventure which would add 10-15 more minutes), you would arrive at the Beach Club in 3-4 minutes. Add another couple of minutes to get to the bus stand. And there is where precision breaks down. No way to know how long you will wait for a bus. But once on the bus, you would be at DS in about 7-10 minutes or so, depending on traffic.
2. Catching a POR/POFQ bus at Epcot's main entrance and then taking the boat over to DS?
This will take considerably longer. Again, counting from the time you leave the Main Gate (as opposed to, say, The American Adventure which would add 15-20 minutes), you would be at the bus stand in 3-4 minutes. And again, there can be no precise answer as to how long you will have to wait for a bus, whether you will have to wait for more than one bus, or if the bus you board will also load one or two
ECV/WCs. (This is less likely to happen at the Beach Club heading to DS, simply because of the difference in the number of people leaving Epcot versus the number of people leaving from the Resort.) Once on a bus, you have a 15 minute ride to POR/POFQ. (If the bus you board serves both places, get off at the first stop which should be FQ. If there are two different buses serving the areas, choose the FQ bus.) From here, you have a 5 minute walk to the boat launch. Then you have to wait for a boat. Could be as long as 20 minutes. Then the boat ride itself is 20 minutes to DS. This would be a fun way to get to DS, but would be by far the longest route in terms of time.
You can also get a taxi at the Epcot main gate exit and that will take you straight there. Total time would be just a few minutes.