Actually, the boat doesn't even touch water until after you are through the tunnel after the drop. During the entirety of the tunnel, the boat rides in a track above the water. Water is actually sprayed at you via water sprayers after you go down the main drop.
Last time I rode Splash was last Thursday and our boat actually got stuck inside the tunnel under the Briar Patch (e-stop, of course). We were sitting in the front row and you could clearly see the track the boat stayed in as well as a stady stram of water flowing under the boat - not touching it at all.
So weight and distribution does not matter. Your position in the boat does, however. If you're in the front, you're closest to the sprayers. Those behind the front row won't get as wet because the people in the front are there to block the spray.