I would not recommend the beaches in the centre of Marbella. However, the beaches around Marbella like Cala de Mijas, El Cable, Puerto Romano and Playa de la Venus are excellent and will make it to the shortlist of best beaches in Costa del Sol.
Just because Marbella has an image of being glamorous and snooty, most assume that many of beaches in Marbella are exclusive to hotels. Sometimes hotels will have access to certain beaches, but people who are not staying in those hotels are allowed in them, so you don’t have to worry about that.
Two of my favourite Marbella beaches are Playa de la Venus and Skol. Skol has a very good chiringuito called El Faro de la Pesquera that you should definitely check out.
Generally, the beaches along the Costa del Sol aren't the golden sand type, unless you go further west towards Cadiz and Tarifa. The beaches in Marbella are generally a little stony, although some actually "import" sand to improve them.
I would say the best beach is outside of Marbella, one called Cabopino Beach. For fun and water sports then Funny Beach is also a good bet and it isn't overly busy even in the Summer.
As for beaches are exclusive to hotels in the Marbella area - some hotels have their own exclusive section of beach only for clients, in other parts the beach hotels or restaurants own the beach beds and parasols. You have to make a purchase or have a meal in the restaurant to use them. Each part of the beach is different. It's usually very easy to hire beach beds and umbrellas on other beaches, but in high season you would need to get there reasonably early in the morning on the more popular beaches.