Hi
You should understand that bathymetry and criterion are 2 different things.
It's possible to use bathymetry (when it' positive) as a criterion or use the bathymetry to build a positive criterion with some mathematical formulae but it's not necessary.
You could build a constant criterion totaly independant of the bathymery.
Regarding the proble you encountered, i think you should take some times to read the Matisse manual or maybe have a look on the tutorial to well understand the way to build a mesh
regards