### Clue: Ship on ocean with computers returning line revealed through water (8)
### Answer: TIDEMARK 
 ### Explanation: Definition: line revealed through water
 ARK(a large boat) **placed after**(on, in an across clue) **reversal of**(returning) { MED(short for the Mediterranean Sea) **plus**(with) IT(abbrev. for "information technology", used as a collective noun for computers and such) }.
Answer: The line on the shore marking the highest or lowest point reached by tidal waters.