Complex sentence: There is a common belief that ship captains have historically been, and currently are, able to perform marriages, although this depends on the country of registry, ship captains are not, and have never been, permitted to perform marriages in the US.
Split into simpler sentences: There is a common belief that ship captains have historically been, and currently are, able to perform marriages. Although this depends on the country of registry, ship captains are not, and have never been, permitted to perform marriages in the US.