Not sure if this will work for your production, but here's how we handled it.

We had 3 wet areas across stage.

1. A lamppost built into a 4x4 platform upside down, polyurothaned and caulked. It was filled with about an inch of warm water

2. A building with a gutter and another 4x4 trough. Water came from a hose from a sink backstage, into pvc with holes cut in it.

3. More water from the hose, this time onto a brick wall with a sidewalk and 1ft gutter in front.

3a. The entire stage was washed with a rotating gobo of water. This was the most visual part of the effect.

The water is at the end of Act 1. We had a good bit of spillage and spent most of intermission out there with towels and a shop vac. We used thin doormats where Don got out of the gutters so he wouldn't slip in the tap shoes.

IMPORTANT. You've got to light the water from behind to be visible. Onstage, our effect was fabulous. From the audience, you couldn't see it. You need to highlight where it falls with some backlight.

This multiple wet spot effect worked well because of the gobo and the illusion of water everywhere. Hope this helps

Jim

Something you might want to consider if building a raked stage that will be getting soaked with water.

Coat the entire floor surface with a gritty additive in your paint (such as sand or silicon).  This will improve traction and hopefully prevent your actors from slipping.

Wow. A raked stage is going to be a huge task. More power to you.

I would advise first putting down some plastic (a dropcloth would be fine). That way if water seeps underneath, it will rest on the plastic and not rot out your main stage between now and the end of the show.

Post some pics!
Jim

I have a question, thinking about doing this show in a few years and have some great ideas for the water...but what about the costumes?? What kind of suit do u use that isn't ruined every night?

Don't know if you've solved your problem yet but....I directed Singin' in the Rain a few years ago and knew if I ruined our new stage floor, I would be dead (my husband is the supt. of the school). We built a platform with a lip on it and lined it with the rubber that they use on flat building roofs. Then we backed it with our scenery and topped it with underground sprinkling hoses. We tested it for my husband and he told us to turn it up!

All water stayed in the platform base and was easy to clean up because the scene is the last one before intermission. Costume was easy too because the suit doesn't get drenched, and he changes out of it at intermission.

The audience loved it-especially the ones that got sprinkled when he swings his umbrella!