…an obvious danger does not make the port unsafe in the ordinary sense, although the loss caused by waiting until it has dissipated is recoverable as a breach of the warranty; and if it cannot be circumvented, the vessel can properly refuse to visit the port. If the master nevertheless chooses to go ahead, in face of the known danger, his action may (but will not necessarily) have the effect of cutting the causal link between the order to the port and the loss.
Per Willmer LJ in Reardon Smith Line Ltd. v Ministry of Agriculture, Fisheries and Food  3 W.L.R. 110 p.152:
Subject, however, to an implied obligation not to nominate an utterly impossible port, the principle is well established that where a charterparty provides a choice of named places for loading or discharge, the charterer is free to exercise his option as he chooses, and in doing so is in no way bound to consult the convenience of the shipowners.
…What, then, is the test by which the permanence of the obstacle is to be judged? The charterers say that nothing short of an obstruction sufficient to frustrate the commercial object of the voyage can deprive them of the right to nominate the port of their choice. On this issue I entertain no doubt that [they] are right.