en_tn/luk/08/23.md

as they sailed

"as they went"

fell asleep

"began to sleep"

A terrible windstorm came down

"a storm of very strong winds began" or "very strong winds suddenly began to blow"

their boat was filling with water

The strong winds caused high waves which pushed water over the sides of the boat. This can be stated clearly. AT: "the winds caused high waves that started to fill up their boat with water" (See: rc://en/ta/man/translate/figs-explicit)