Lots of great advice here! I am impressed. The bottom line is that you don't really know. I have ridden that ride every visit for 12 years at least once per trip. It's my favorite ride in the parks, and it never gets old. I have had every possible seat position, and ridden in every possible weather condition. Sometimes you get a spray, and sometimes pretty wet, but never completely drenched like Kali river rapids, or Dudley DooRight over at Universal. Sometimes the person in front of you takes the hit, and other times they spend every possible water moment crouched down below the lip of the boat and let everything fly into your face. You just don't know. If it's cool outside wear a poncho. Put plastic under your bottom to avoid the water in the seats (its not usually too bad unless it was empty the ride before). If it's hot outside then you are never drenched too badly to be uncomfortable. But for goodness sake go. It's one of the most magical attractions every made. Happy and musical and colorful and wonderful and not the least bit "sappy". Disney spared no expense and tossed everything possible into this ride. Worth the admission to the Magic Kingdom all by itself. (OK, maybe I am going a little over the top, but it's really great!) I admit my kids prefer Space Mountain for the rush, but everyone in the family can agree that Splash Mountain is a masterpiece, and one of the things that makes Disney....."Disney".
Advice from a 47 year old Disney fanatic.