First, I guess you're using the cloth plane or another multi-polygon surface for the water.  The reason I say this is because the artefacts you're seeing are due to Poser shadowing the polygons.

You can cure this by adjusting the Shadow Bias on the lights and the object itself. To adjust the ones on the object, take a look at the Reflect Node. Ziggie posted a solution some time ago which went like this: Quality - 0.1, Bias - 2.0.

You could also use the Single Sided Square instead of the water plane you're using, unless you need modelled in features such as waves and ripples, which can be simulated by displacement anyhow.

Coppula eam se non posit acceptera jocularum.

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