Written permission is required prior to using the likeness of a person or their identifiable property for commercial gain. In the case of a swimwear model, the model must sign a consent form, ideally for each image, before the images can be sold for use on a poster, in an advertisement or elsewhere. Images of property which can be identified as being owned by a particular person may also require release. News images may be regarded as exceptions to the rule in many jurisdictions.
Whether or not the model is in a public place, or is in any case well known and often photographed, makes no difference if images are used commercially. A photographer can be sued if a release has not been obtained for a commercially published image..
Some models will ask for payment before signing a model release form. make sure this process is understood and agreed by both sides before selecting or hiring a model. Others may be happy with a decent print or just to sign the release as a favour. Whatever the situation, discuss it openly and get clear agreement. Then file the signed form carefully for future reference. Images are of no commercial value without a signed release.
Paid professional models are generally expected to sign a release before the shoot, and most will understand and be happy with this procedure. Model agencies often have their own model release form for this purpose. However in the case of nude or topless work the model may quite reasonably wish to see the images before signing a release. A photographer must see the situation from the model's point of view as well as his or her own.