In Corfu, (Kerkyra) you can book a hotel and use its swimming pool. You can also roll up to any other swimming pool and use that, free of charge.
Because you can use any swimming pool of a hotel or accommodation that has one, you can use the best pools on the island, free of charge. - back up here
I believe this rule is the same on other Greek islands, does anybody know specifically where these rules apply? Are there similar rules in other tourist destinations?