Draw an horizontal line somewhere in the middle-ish of the triangle, showing the height of the water. Label the height as "h" and (half of) the width as "w". Note the nested, and thus similar, triangles. Use this to solve for h or w in terms of the other variable.
Use the formula for the area of a triangle to create an expression, in terms of the other variable, for the cross-sectional area. Then multiply by the length to get the volume of water.
If you get stuck, please reply with a clear listing of your steps and reasoning so far. Thank you!