Be sure to visit the touristy site just beyond the town of Paia and Mama's Fish house going toward Hana. It's right off the highway and well marked. There's a hillside overlook where you can park, or you can drive down the road close to the water to get a close up look and talk to some of the windsurfers. Many are pros from all over the world. Watch and marvel at their forward, backward and double loops!
Now, an even better way is to go to Kanaha Beach Park. It's very close to the Kahului airport. Just drive around the airport loop, take a right turn where the car rental agencies are, go to the end of that road, turn right again and go to the end where you will find the park. Kanaha Beach Park has several entrances, take the last one. You will find good parking, rest rooms, etc. Take a SHORT walk down to the ocean where you'll find a super beach (even for swimming) along with wind and Kite surfers. It's fun to watch them rig up and sail off. You're right up close, so you can get a first had look and photo of some expert sailors performing seemingly effortless jibes.