How much of the world is deeper than X

I was asked for fraction of the ocean deeper than 20 fathoms. That gets tricky. What would the differnce be if this were at MLLW vrs MHW? I welcome input from anyone specializing in geodesy, vertical datums and tides.

I prefer to work in units of chains. So how much is deeper than 1.8 chains... 97.47% of the WORLD (oceans + other things that might be deeper but not in the ocean). And this is relative to whatever vertical datum topo30 uses.

Here is using the topo30 2012 edition from David Sandwell and JJ Becker. First a warning. There is a non-commercial license on the topo30 data. This is more restrictive than the GPL license that many are familiar with.

Permission to copy, modify and distribute any part of this gridded
bathymetry at 30 second resolution for educational, research and
non-profit purposes, without fee, and without a written agreement is
hereby granted ...

... use for commercial purposes should contact the Technology Transfer
& Intellectual Property Services, University of California, San Diego ...

wget ftp://topex.ucsd.edu/pub/srtm30_plus/topo30/topo30.grd
gdal_translate topo30.grd topo30.tif
grdinfo topo30.grd

topo30.grd: Title: topo30.grd
topo30.grd: Command: xyz2grd -V -Rg -I30c topo30 -Gtopo30.grd=ns -ZTLhw -F
topo30.grd: Remark: 
topo30.grd: Pixel node registration used
topo30.grd: Grid file format: ns (# 16) GMT netCDF format (short)  (COARDS-compliant)           
topo30.grd: x_min: 0 x_max: 360 x_inc: 0.00833333333333 name: longitude [degrees_east] nx: 43200
topo30.grd: y_min: -90 y_max: 90 y_inc: 0.00833333333333 name: latitude [degrees_north] ny: 21600
topo30.grd: z_min: -11609 z_max: 8685 name: z
topo30.grd: scale_factor: 1 add_offset: 0

# install ipython, gdal-py and numpy
ipython

import gdal

src = gdal.Open('topo30.tif')
topo30 = src.GetRasterBand(1).ReadAsArray()
num_deep = (topo30 < 36.576).sum()  # 631309778
'%.1f %% of the globe' % (num_deep / float(topo30.size) * 100)

67.7 % of the globe

num_ocean = (topo30 < 0).sum()
'%.1f %% of the ocean is deeper than 36.576m' % ( 100. * num_deep / float(num_ocean) )

'97.5 % of the ocean is deeper than 36.576m'

Minus issues with resolution, vertical datums, tides and other stuff, 97.5% of the world is