For coastal areas I would always suggest
Brabantia post boxes or if you need something quite large,
Bobi post boxes. Both brands offer excellent protection from the corrosive salty air and will not leak.
You might have a problem under extremely windy conditions as the letter flap may blow open. You could solve this by attaching small magnets to the letter flap or choose the product 480829. Rather than a single metal flap, this model has a series of plastic flaps that are designed to resist wind.
Anthony N