I'd suggest I-90 (Boston to Seattle), though I'm not sure of all the details of what happens going through Chicago.
Note that staying on I-90 isn't actually the shortest route from Logan Airport to Safeco Field; if you ask Google Maps it'll put you on a few other highways along the way, cutting off some of I-90's corners.
But you're definitely going from a tunnel under Boston Harbor (connected to the Atlantic) to less than a mile from Puget Sound (connected to the Pacific).
Note that staying on I-90 isn't actually the shortest route from Logan Airport to Safeco Field; if you ask Google Maps it'll put you on a few other highways along the way, cutting off some of I-90's corners.
But you're definitely going from a tunnel under Boston Harbor (connected to the Atlantic) to less than a mile from Puget Sound (connected to the Pacific).