In Chapter Four of the book by Jack Davis, navigation in shallow Gulf waters around the Florida Keys was described by Bernard Romans as "mazy", or maze$-$like. Which of the following contributed to tricky navigation in these waters? More than one answer may be correct, please choose all correct answers.

a$.$ Coral reefs covered most of the bottom and often damaged boat hulls.

b$.$ Massive oyster reefs were major hazards, as they sometimes stretched for miles.

c$.$ There were so many wrecked ships that a captain needed to weave through.

d$.$ Ships could run aground on sandy shoals miles out from shore.