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 

Open new DEM  for manual opening of DEMs, pick the HDR file.

Last revision 2/19/2014