What color are the circuit boards? Prototypes usually have Red instead of green

It's not a prototype game, but it could be a sample game. Typically in a production run, there can be about 5-15 prototypes (usually with an "X" in the serial number and red PCBs), sometimes 25-100 sample games, then followed the rest of the full production run.

P-1 is certainly early game code.

If you replace the game and sound ROMs with EPROMs that have the latest code, it should play like typical whitewater.

Here's an example of an actual prototype whitewater: https://pinside.com/pinball/forum/topic/white-water-prototype-or-sample

That depends on the game title. Sometimes, there's not much of a difference, sometimes there are quite a few differences. Artwork changes, parts placements, adding/removing/changing mechs, etc.

With whitewater, there were some minor artwork differences, but I think that was about it.

Sample games were typically tested on location to catch any final issues before the rest of the games were built. Depending on the feedback from operators, sometimes there were minor changes between sample games and production games, but sometimes there were no differences.

My good friend has WH20 HEP serial 0001 and that P-1 code is still on it. Actually kinda fun!