ive done it. DO NOT take a large raft that sits high up out of the water. we had a big raft one year and it took us FOREVER to get to the finish, and we kept getting blown out to the shipping channel/Canadian side. ended up getting severely sunburned, even with a ton of sunscreen.
take water in your coolers too. everyone packs them full of beer, about hour 6 the only thing you want is a drink of water. lol.
best way to do it is individual rafts, and tie them all together, where you are down in the water. the current will take you that way, we were riding up on top of the water. no good.
also, its a long day. people have been getting off early in recent years and IMO that makes the trip better, there are a few places along the river that have big float down parties, getting off there is my suggestion. going all the way to Marysville is a trip! we all ended up swimming a long way the year we had the big raft, pulling it back in from the shipping channel every 20 minutes lol.
you will have fun though!