Dvucca Isle Ilrusi Atoll
Easier for me to link it than try and explain it all.
Nyzul Isle is the only one I know of that uses the undersea ruins.
And as I recall, Ilrusi is a bitch to get. Soulflayers suck. Plus it just takes awhile to get there.
EDIT: K, I didn't do dvucca the same way OMG did it. Didn't have the other portal at the time. Atleast I don't think I did. Its been too long.