GOD'S WORD® Translation
When they saw him walking on the sea, they thought, "It's a ghost!" and they began to scream.

American Standard Version
but they, when they saw him walking on the sea, supposed that it was a ghost, and cried out;

Berean Study Bible
but when they saw Him walking on the sea, they cried out, thinking He was a ghost—

Douay-Rheims Bible
But they seeing him walking upon the sea, thought it was an apparition, and they cried out.

English Revised Version
but they, when they saw him walking on the sea, supposed that it was an apparition, and cried out:

King James Bible
But when they saw him walking upon the sea, they supposed it had been a spirit, and cried out:

World English Bible
but they, when they saw him walking on the sea, supposed that it was a ghost, and cried out;

Young's Literal Translation
And they having seen him walking on the sea, thought it to be an apparition, and cried out,

Mark 6:49 Additional Translations ...