Complex sentence: They were not a truly marine species, being confined to coastal waters; however, the sea mink was unusually aquatic compared to other members of Musteloidea, being, next to otters, the most aquatic member of the taxon.
Split into simpler sentences: They were not a truly marine species, being confined to coastal waters. However, the sea mink was unusually aquatic compared to other members of Musteloidea, being, next to otters, the most aquatic member of the taxon.