Or simply "underwater"?

it's most definitely not "underwater". "underwater" is used as an adjective, as in underwater camera, or refers to a realm below the surface of water. since the original also mentions conditions, I would translate simply as "in under water conditions"

An investigation on the compresibilty and setlement duration of a lateritc soil ofYaoundé, Cameron, was performed to check on its ease to cary loads of civilenginering structures. The increase in setlement with water content is more acentuated with a stronger load than a weaker one. The compaction also considerably
decreases the compresibilty of these soils, rendering the action of water almost nul onthe variation of setlement except when the compaction is not madbe with the optimum Proctor. These signifcant observations compel us to consider only, for a lateritc soil or
another soil, the compresibilty characteristics under immersion conditons. The findings of this work agre with the principle of the odometric test described in the FrenchStandard NF P 94 90.

British Journal of Applied Science & Technology
4(21): 303-301, 2014
SCIENCEDOMAIN international

http://www.sciencedomain.org/abstract.php?iid=541&id=5&aid=4...
Sustainable Study of Local Lateritic Soils Compresibilty
Robert Medjo Eko1, Lezin Seba Minsil2*and Etone Maka Alexandre Dodo1

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Note added at 3 days8 hrs (2014-08-10 16:47:07 GMT)
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"immersion" is inside water, not necessarily under water (total immersion).