"There's no coral reef below 30 metres.". ..... thats a bold...

"There's no coral reef below 30 metres.". ..... thats a bold statement. it does depend though on pone's ability to go down deep enough to find the corals that do live deeper than 30metres.

but yeh, the GBR most commonly seen corals are down to 30metres.

but as to water temperature, simply measuring the surface temperature doesn't reflect the consequence of GW. and do you know the exact site(s) that Cook took the temperature? the depth as well as location? was this at a time/season when the equatorial flow was moving south? was it La Niña or El Niño at the time?

so many factors sailorfnq.