Regular, exterior-grade plywood should be fine - painting it or coating with water sealer would be a good idea - The only place you'll probably have a problem is at the base of the pipe - water may collect there. You may want to use a better grade of wood there. Also, squeegee it off after it rains.
The sides of the pipe should be alright - the water will run off (the seams are where you'll want to make sure to really seal well - paint, tar, liquid nails, wax - anything to repel water.)
It's just a skateboard ramp - it should last a few years before you have to replace a piece of plywood here and there.