One way you could do a rain splash bump is to take a top view render, and creating the splash bumps over that render, you could use the outline of the ship as a guide as to where not to make splash bumps.

I used the the same technique for a woden stool sitting in the rain. In addition, I also top rendered out where the legs of the stool would be placed in a shallow puddle, so I could make a bump map for tiny waves emanating from them.

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