Amban Beach is the most accessible swimming beach near central Manokwari, a short ride by ojek (motorcycle taxi) from the city centre. The water is calm and relatively clear, making it a popular spot for an afternoon swim. Vendors sell food and drinks along the shore on weekends, when local families come out in force.
The beach is nothing spectacular by Indonesian standards, but it's convenient and pleasant. The bay views across to the islands are a bonus, and on clear days you can see Mansinam Island offshore. If you want better snorkelling, ask locally about further beaches along the coast or arrange a boat to one of the bay islands.