Dar es Salaam, Tanzania, is not a city in which most American travelers will wish to linger. Although not actively dangerous like Nairobi, it’s a sprawling, ramshackle mess, often with diabolical traffic. Twenty years ago, the harbor could be quite picturesque, thanks to the sailing dhows that used to trade up the east coast of Africa to places as far afield as Bahrain and Dubai. But nowadays marine transport is chiefly provided by huge Chinese container ships.