DIY project help

[causearuckus]

I am a water polo player and am trying to end the age old debate of the ball "floating" into the goal. To make a long story short, to score a goal the whole ball must enter the goal, not just part of it. I want to build some sort of device that can "sense" the ball in the goal, and signal it with a kight or buzz like in hockey. I dont know where to start sensor wise, but im thinking something inside the ball. I dont know how much motion sensors would work because of all the water flying around and the goalie's body ends up in the goal a lot. And all the stuff needs to be water resistant/proof. And ideas for what to look for?