Jamieson, Fausset and Brown on Hebrews 11:29

29. they--Moses and Israel.

Red Sea--called so from its red seaweed, or rather from Edom (meaning "red"), whose country adjoined it.

which ... assaying to do--Greek, "of which (Red Sea) the Egyptians having made experiment." Rashness and presumption mistaken by many for faith; with similar rash presumption many rush into eternity. The same thing when done by the believer, and when done by the unbeliever, is not the same thing [Bengel]. What was faith in Israel, was presumption in the Egyptians.

were drowned--Greek, "were swallowed up," or "engulfed." They sank in the sands as much as in the waves of the Red Sea. Compare Ex 15:12, "the earth swallowed them."