Unfortunately, there is no simple answer to this question..
Location in the car matters, but not all of the time.. weight of the car (people) matters a LOT.... but the BIGGEST issue is weight distribution... REGARDLESS of the max weight in the boat, if the weight is not distributed evenly, then the front row will get soaked.. for example..
If there are fours adults and four small children getting into a boat.. if the majority of the adults sit on the right of the boat, and the small children on the left , then the boat will list a bit.. even an inch or two is enough to give the boat a shorter plane on the heavier side, thus 'cutting' into the water on that side.. and generating the 'wave' on that side of the boat..
thats the #1 reason people get soaked on splash mountain... otherwise you get a chest up spray across the whole car... If you are in rows 2-4, the previous rows will buffer you from the spray.. Unless of course, they duck..... which is why my wife no longer rides in front of me
..
Also... here's the WORST possible scenario.. the boat has a list to it, and you are in the front row.... DO NOT DUCK! take the chest-up spray like a man (or a woman) and then let the wave do it's worst on the rest of your front side..
If you DUCK in this scenario, you'll get the wave of water down your back, backside, and legs......... making it worse..
Unless of course you WANT to get soaked... then ignore my advice
trust me.. I've ridden Splash roughly 30-40 times in the last couple of months..