Well if the water was coming from the closed-loop part of the cooling system, it would be the same color as the antifreeze and you would notice that the level in the expansion tank has dropped; you'd also be experiencing engine overheating.
If the water is coming from the open-loop part of the cooling system, that's easy to see. When I had a problem like this I took my ski to the boat ramp and ran it with the seat and seat support removed. With a flashlight, I could see the water was leaking in from the center O.P.A.S valve. The gasket was leaking.