I have kept black mollies in FW and marine aquariums. Scats, monos and some puffers will do too as will bumblebee gobies. Most livebearing tropical fish such as guppies and swordtails will appreciate salt in their water.
In mu local river which runs from Newhaven to Lewes (river Ouse) in East Sussex has sea trout, bass, mullet, dabs and a few other sea fish in among the dace and cream and carp.