Among beaches with timeshares, I would not stay in Kihei: you need to cross the road to get to the beach and the area does not look nice. The Westin timeshares north of the rock have a good beach, but it can be rough if windy. I would personally choose to stay at the Marriott south of the rock. Yes, the beach is rocky directly in front of Marriott, but it has fantastic snorkeling, and you could walk a short distance north towards the black rock where the beach is sandy. Plus, I like the area around the Whalers village a lot more than the area around Westin timeshares where you are mostly confined to the resort with much less to do.
<---North...............Sheraton Black Rock..................South --->
Ka'anapali Beach is the beach to the right (south) of Sheraton Maui (fka Sheraton Black Rock), which is that resort that sticks out like a thumb in the photo above, however tour guides, etc pretty much group both sides into one area they call "Ka'anapali." Ka'anapali Beach (south) is the original resort area of Maui, and what I call "resort row." It's similar to Wailea, where you have hotels all lined up next to each other along an ocean front path. Where in Wailea you'll find Four Seasons and Grand Wailea resorts, here you'll find the Westin and Hyatt hotels, as well as timeshares like Marriott, The Whaler, and Hyatt. Marriott and The Whaler are in the middle of the action and the Hyatt is at the very top (on the map) of resort row. The oceanfront path is great for walking or jogging to neighboring hotels/spas, so if you're into strolling, this is the place to be. The downside is that it can get pretty crowded. The area in front of Black Rock is known for good snorkeling, but you'll have to walk or swim a ways to get there.
The area left (north) of Black Rock is where you'll find WKORV. Surrounding it are a few older, low rise hotels and a bunch of condo communities. Because it is more condos than hotels, it's quieter and your options for dining are pretty limited if want to walk there. However, WKORV has a free shuttle that will take you to the Sheraton, Westin, Whaler area and Lahaina for free, or, of course, you can pay for a taxi/shuttle to Lahaina, which is about 10-15 minutes away, depending on traffic. There are other timeshares as you go north into Honokowai and Napili, but generally speaking, as you go north, it gets quieter.
Personally, I'm more comfortable being "near the action" than IN the action, so WKORV is my preference, but it's really up to the individual.
If you're into swimming, Hawaii isn't really conducive to swimming unless you're an Olympian, because of the strong currents and wind. You won't see a lot of swimming except in pools, and all water sports (snorkeling, kayaking, and swimming) are best done in the morning before the trade winds make it over the mountains. If you want to lounge on a beach and sun yourself, there's no better place to do it than Hawaii because it's almost always the perfect temperature, along with a lovely breeze.
If you want to lounge on a beach, most resorts won't allow you to drag their lounge chairs to the beach, but most will rent them to you for a fee. Or you can rent chairs from rental agencies by the week for a much better price. For $20, I always rent a Tommy Bahama chair and umbrella for the week. We love beach hopping. Makena has been mentioned, but I'll also mention Ho'okipa on the other side of the island, which is great for lounge lizarding while watching the surfers and kite surfers.