okay...
I voted for passing, because it's nifty and gives me nostalgia for Pipe Dream, lol. Here's a few easy improvements you could make:
1)---Music. This has a very small filesize, so you have plenty of room to stick some nice music in there! Try to find something appropriate to a fast-paced puzzle game and it adds a LOT to the overall quality.
2)---Background. Right now, it's so...utility room, lol. All bland. Find a good background picture, add some color to things.
3)---Score. Make a score option for the more pipes the water travels through, and make you lose points for replacing a laid pipe piece, lol.
I wish I could give you more but I'm a total newbie at flash, whose skills are limited to tweening, lol. But I'm sure others can help ya get better.
This is playable and passable, but could use some extra spice, lol. Good luck in the future, and I hope this passes judgement.