:::Thanks Karl, I just got back from a camping trip and hadn't gotten to responding. On the Geospatial Data Gateway downloads the projection choices are, Geographic, State Plane, and a couple types of UTM, I think. Geographic is Equirectangular projection, sometimes called "unprojected", or Equirectangular projection, and is widely used. In theory it shouldn't matter as long as one's datasets contain info about what projection they are in. The downloaded data from the Geospatial Data Gateway does, so it shouldn't matter too much what projection you choose. Still, "geographic" seems a generally safe bet. The formats they offer, at least for the watershed boundary dataset, are "ESRI Shapefile" and "ESRI Geodatabase". I picked shapefiles, which worked fine in ArcExplorer. I'm not sure if ArcExplorer can deal with geodatabases. ESRI invented both formats, I believe, but shapefiles have been around longer and the format more widely supported. As for base maps, ArcExplorer seems to come with a bunch of general purpose datasets, like US state boundaries, counties, and so on. I found it useful to use state and county boundaries along with the WBD, to help orient myself. The display of state and counties are "filled polygons" by default, using a random color for the fill. In order to view states, counties, and watersheds (all polygons), you need to turn off the fill colors for states and counties. In ArcExplorer you do that by right-clicking the layer name (control-click in Mac OSX) and selecting "Properties" from the popup menu. The fill and outline color choices open up. I made the states and counties "transparent fill". There's various ways to color things to show different aspects of the dataor just use a default single color. There's also a "Labels" tab under properties. With WBD polygons, I found it helpful to set "label features using" to the "HU_10_NAME" field. The watershed names are then displayed in each polygon (unless you are zoomed out such that the labels would overlap, in which case some are not shownbut you can always zoom in, or set the font smaller). 

 :::But as Karl said, you don't even need to make a map or fiddle with display setting if you don't want. If you right-click on the WBD layer (it downloaded with the name hu5, for 10-digit basins for me), and from the popup menu choose "Attribute table", you can view the data in table form. Fields include "ACRES". It took me a while to figure out which basins were relevant. Sometimes I wasn't sure unless I opened other datasets, like NHD stream lines, and turned labels on, zoomed in, checked what I was seeing with topo maps, and so on. At first it can be a little tricky to get the hang of it, but it gets easier the more you do it. Anyway, must put kiddo B to bed.