We just got back from WDW and my 4 year old DD loved this ride so much that we rode it at least 8 times.
You really can get wet in three places - first, in the area next to the big drop, second, on the big drop, and third, at the end of the ride next to a waterfall.
I do not think it matters if you sit in front or back - the front may get a bit wetter on the big drop but the back tends to get hit harder at the other two places.
As to the weight issue - I do not think this matters either - was on the ride more than once when large people were in front (including me) and did not see any difference in how wet you got.
The biggest change that WDW can make to prevent getting to wet is to alter the timing of when the boat passes the area next to the big drop and also the amount of water going over the waterfall at the end. Since we were at WDW at the end of Jan. during record cold snap, the ride operators did make such changes and on several occasions we did not get very wet at all.