The SOP that I've written into our Operating Manuals is that if the runway is less than dry, up to the maximum permissible water depth for a "Wet" Takeoff, both the Dry runway, and the Wet runway Takeoff Weight limits be obtained, and the aircraft operated at the lesser of the two as the RTOW. (There are
SOME circumstances where, because of the Reverse Thrust and reduced screen height concession, it's possible to derive a wet runway RTOW higher than for a dry runway).
The JARs do require such a consideration for wet runways (but a grey area remains for damp runways), but not all regulatory authorities impose such a requirement.
For the record, we also consider a dust or sand covering on the runway as a wet runway, and apply the same operating policy specified above.
A bit of JAR, a bit of Boeing, and a bit of our own thinking, that's the way that it was submitted it to the regulatory authority for compliance for the AOC, and it met with their full approval.
Regards,
Old Smokey