Sidwell has a tennis camp that spends at least some afternoons (don't remember exactly) swimming.

You could do morning tennis / afternoon swim at St. Albans. It's a full day (9:00-3:30) and might be a little more intense than you're looking for, but the instruction is really strong for both tennis and swimming. Our older DC did a couple weeks of the tennis / swimming combo two summers ago and the leaps forward he made in both sports were impressive. (By contrast, his swimming has never noticeably improved at Valley Mill or Beauvoir or anywhere else where the day includes a "lesson.") He and our younger DC both will do a week of tennis / swimming at St. Albans this summer.

Landon school has a day camp with swim and tennis. How old is your kid? The day camp is for kid under 10 or 11.

Lafayette Tennis camp has full day and half day options. I believe 2 days a week they go to a local pool but not a swimming lesson. I know Casa at Sheridan offers tennis and swim as its course offerings, but that is a full day camp and the tennis/swim would be a small component.

Look at the YMCA in Bethesda. The tennis camp offers swim but it may not be a swim lesson- it may just be free swim

I'm looking to fill the first week of the summer break with a tennis and swim camp - basically a low key camp with an hour or two or tennis instruction followed by a swim lesson (these types camps abound in the Philly area where we are from, but I haven't found any online and everyone I ask only seems to do full day camps which would be all tennis or all swim). Kids are 9 and 11. Thanks for any suggestions.