In short
Sea temperatures are under Weather at /weather/sea-temperature/… (pick your state slug). Classic seatemp.asp links are gone.
Sea temperature pages live under Weather:
/weather/sea-temperature/western-australia (and the same pattern for other states, e.g. new-south-wales, queensland).
You can also reach sea temperature from the Weather menus when the product is enabled. If the page says it is temporarily unavailable, that is a site-wide switch — try again later or use Feedback.
The old *_seatemp.asp graph links are retired.