shoal

1. a large number of fish swimming together.

   a shoal of bream

1. (of fish) form shoals.

   these fish can safely be released to shoal with most adult species

late 16th century: probably from Middle Dutch schōle ‘troop’. Compare with school².

1. an area of shallow water.

   we clawed our way out from the Bahamian shoals into the deep waters of the Atlantic

1. (of water) become shallower.

   the water shoals reasonably gently, and the swimming is safe

1. (of water) shallow.

   crabs will move to shoaler water when the current changes

Old English sceald (adjective), of Germanic origin; related to shallow.