im into a robotics competetion tat asks 4 building a bot tat plays water polo

the dimensions of field r 4m*2m. depth of water is 7 cm. Im thinking of making an underwater car like thing wid propellers as well.
it will b abt 5 cm tall n 20 cm long

the prob is i cant think of a gud mechanism 2 catch the ball(TT)
i was thinkin of usin a rack n pinion system tat cud b used 2 push a net up n down(as net goes up, water drains out n ball is caught

but i feel tat may b complicated.
pls help