What is the ruling on buying a house and renting it out in a coastal city, and on seasonal work selling food, phones, and swimming gear to people who go to a sea where there are objectionable things?

The basic principle is that it is permissible to rent out a house to vacationers and to sell permissible items, unless it is known that the renter will use it for forbidden acts, such as drinking alcohol, or that the buyer will use the permissible item for forbidden purposes. In such cases, it is not permissible, due to the involvement in cooperation in sin and aggression.