Lost at Sea at the Folger Shakespeare Library (free; no tickets required). This well curated collection of artifacts and illustrations emphasizes the uncertainties of watery horizons, and the possibilities they offer for creativity. Steve Mentz, author of At the Bottom of Shakespeare's Ocean and the curator of the exhibition, will be lecturing on Stories from the Sea: At the Bottom of Shakespeare's Ocean on Tuesday July 13. The event is free.
If you're not in DC, the digital exhibit is quite good as well. Just follow the link above. You may also be interested in this review from the Washington Post.