A few minutes later Tom was in the shoal water of the bar, wading toward the Illinois shore. Before the depth reached his middle he was half way over; the current would permit no more wading, now, so he struck out confidently to swim the remaining hundred yards.
-- Mark Twain, The Adventures of Tom Sawyer, 1876
That piece of land is permeated with a perfect shoal of mineral springs.
-- Ada Clare, Only a Woman's Heart, 1886
Shoal comes from the Old English term sceald meaning "shallow." It was spelled with a d up until the 1500s.