I really want to ride these when I go in June! I'm just afraid it's going to be sooo hot and I'm not gonna want to row all around the island!

Seems like a lot of fun tho. Really afraid of falling out tho...I can swim very well, but it seems like it would tip over easily.
You don't typically row nonstop (that happened once to me when the guide decided that he'd rather talk to his buddy sitting right behind him instead of doing his job

). Usually, it's row a bit, talk a bit, row a bit, talk a bit, and then row the last stretch from the loading area from the Mark Twain to the loading dock for the canoes (because of the Fantasmic stuff, the canoes cannot continue to hug the island shoreline so you have to skedaddle to keep out of the way of the big boats). Not everyone has to be rowing all at once for it to have momentum (the water in the river does have a current, thank goodness for that because most people tend to give up at one point or another, especially kids who typically row for the first 1/4 and then sit back and relax for the rest of the "ride"

). I have never heard of a boat tipping over, and even if that were to happen, the canoes are usually so close to shore (to keep out of the way of the Mark Twain and the Columbia) that you'd probably be only in the water for a few minutes before you're walking on the bottom up to the shore. As for the heat, sometimes you are in the shade and, now this is gonna sound gross but I've been in more disgusting water so it doesn't phase me, since not everyone really knows what they're doing you can and may very well be splashed by water that is projected upwards by paddles. Just keep in mind that all of the water is on the same system, so you're getting splashed by the same water that gets you wet on POTC and Splash.
It's something that you've gotta try at least once, unless you're deathly afraid of drowning, in which case, I highly recommend that you do not put your sanity at risk and ruin your day at the Happiest Place on Earth and opt to stay nice and dry on shore.
