Hi JxBx,
I ran into this same issue quite some time ago and still haven't found a solution to it. For me, the best next thing was to create a shell element spanning the length of the tank at the height of the expected wave and then verifying it visually (do not assign any contacts to the element, just use it as a reference guide). Obviously it does not give an exact answer but if you are verifiying your results against a paper you should be able to tweak it over several iterations to get a match.
On a side note, do you mind telling me the EOS along with the values you are using for water? This is something I have been struggling with for quite some time now.
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