SRTM-30 is clearly the best of the 30 second (about 1 km horizontal resolution ) global DEMs. The only problem might be excessive elevations in urban areas because of buildings.
Consider using ETOPO1, which includes the oceans, or now SRTM 30 Plus.
Download from USGS site
you need the zipped HDR and DEM files
files are named for the NW corner
unzip and get the two files into the same directory
Open new DEM for manual opening of DEMs, pick the HDR file.
Last revision 2/19/2014