It's a 10-15 minute drive up 395 from DCA to the ballpark.
You could take the water taxi from Old Town Alexandria to the ballpark, but that would involve both Metro and a bus ride between DCA and Old Town, and would probably take a lot longer than taking Metro alone. And Metro shouldn't take more than 20 minutes or so during the week. On the weekend, that's another story.
How much would a cab cost from the Airport? And the Old Town Water Taxi is expensive, for 1 person, I suppose it isn't too bad, but with my wife and gets, it would be pricy.
