Since you will not be disturbing the natural environment or removing anything natural from the beaches, beach litter surveys will not be damaging to the environment. You are helping keep the environment clean and healthy!
For this reason, it is generally not required to get special permits to sample at your local beaches. However, the sites you choose may be protected or may have limited access to the public, so you need to make sure that you have permission to sample at these sites and that you carry that proof of permission (e.g., a permit) on you while you are at the site.
Sites, where you may need to get permission to sample, include sites in nature reserves or other protected land, sites on private properties, and sites outside of your home country. Your county may require you to have a research permit if you are collecting data. To find out whether you need permission to sample at a site, you will need to do some research. Contact the appropriate authorities (e.g., councillor or nature reserve management) to confirm that you have all the required permissions to sample at your site. In some instances, you may need to obtain a permit to conduct your surveys in a particular area. Permits are not only necessary to do research in a country or a certain area, but they might also be required for access to certain private lands or beaches.