Pangani is a sleepy small Swahili town at the coast south of Tanga at the outlet of Pangani river. It has nice beaches.A number of hotels are said to have mushroomed in recent years.
The mangrove forests are also particularly extensive here. There is a fantastic beach 16kms south of Pangani called Ushongo Bay.
It is possible to swim all day as there is water even at low tide. I can recommend The Tides Lodge, it has great food and has great beach side cottages.
From Tides Lodge is possible to snorkel and scubs dive on Maziawe Island marine park which is has excellent marine life.
The Tides can arrange transfers to Saadani and Pangani, and even has it's own airstrip if you want to charter your own plane to Zanzibar or Dar es