For diving, they go on a slope with a minimum of 3.0 m to ten.0 m (10 to 16 feet) all of the deep closing stages. These marks on the mask is needed align the laser markings for the radiation treatments. There are many types of dhows but the most commonly encountered ones are the shu'ai and the boum. https://Near-Finder-Australia.com/en/business/wa/henderson/diving-services/franmarine-underwater-services_2547345+4.html