Well, I dont run my RC underwater But I do run it through many a puddle and some deep snow.
Theres 3 ways I know to seal your servo, best yet do all 3!
1. loosen the 4 screws off from your servos. Take some clear silicone like the type that is used on glass fishtanks etc. I guess colored silicone would work fine too. squirt a little bit all around both of the cracks of your servo and then push it into the cracks with your finger. TIghten the screws back up and let it dry. also get soem where the cord comes out. Then you can cut off the excess silicone.

2. take a baloon and put your servo inside it and have the cord comming out of the baloon hole. zip tie the hole tighly shut around the cord. then cut a small hole and stretch it so your servo drive(where the arem attaches) is sticking out.

Also, you will want to seal your batter/reciever case. I did this using silicone again. I just opened it up and made a bead of silicone all arount the outsied of the reciever box lid and let it dry while it was open. This made a nice fitting gasket. Then where the servo vords come out of the box, I put enough layers of shrink wrap on the cords at that spot so that it fits very tightly. Then i used more silicone [>:]

3. Take your servo and Dip it in that plastic coating you can buy for doing tool handles. Cant remember the name of it but there are a few brands. It dries like rubber.

As far as a snorkel goes, it would be easy to do with either some stiff pipe like pvc or copper, or with some flexable pipe likesome small radiator hose. make sure you mount it so it wont get knocked off if your truck flips though.
And remember, when you get your truck deep into water or snow, the engine will cool from running temp almost to the water temo instantly. This is quite bad for it. I stil do it though