People seem to have grasped the fact that there is huge amounts of water/ice at high latitude locations, say above 60 degees latitude north and south of the equator according to the GRS instrument on Mars Odyssey.
But many still quite haven't caught on to the fact there is substantial water/ice at some near equatorial sites as well including Gusev, WITHIN CENTIMETERS OF THE SURFACE:

VERTICAL DISTRIBUTION OF SHALLOW WATER IN MARS SUBSURFACE FROM
HEND/ODYSSEY DATA.
I.G. Mitrofanov1, M.L. Litvak1, A.S. Kozyrev1, A.B. Sanin1, V. Tretyakov1, W.V.
Boynton2, D.K. Hamara2, C. Shinohara2, R. S. Saunders3, D. Drake4, 1Space Research Institute, RAS, Moscow, 117997, Russia, imitrofa@space..ru, , 2University of Arizona, Tucson, AZ 85721, USA, 3NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA, 4Lansce 3, Los Alamos Nat'l Lab. Los Alamos, NM and TechSource Inc, Santa Fe, NM 87594, USA.
http://www.planetary.brown.edu/planetary/international/Micro_38_Abs/ms069.pdf

VERTICAL DISTRIBUTION OF SHALLOW WATER IN THE DISTINGUISHABLE REGIONS AT LOW AND HIGH LATITUDES OF MARS: NEUTRON DATA DECONVOLUTION OF HEND.
I.G. Mitrofanov, M.L. Litvak, A.S. Kozyrev, A.B. Sanin1, V. Tretyakov,
W.V. Boynton, D.K. Hamara, C. Shinohara2, R. S. Saunders, D. Drake, R.
Kuzmin
http://www.lpi.usra.edu/meetings/sixthmars2003/pdf/3080.pdf

Perhaps as much as 10% water by mass or 15% to 20% by volume.

Bob Clark