What aquatic ecosystem is likely to be the warmest?

Coastal at the temperate latitude, basically the deeper you go, the colder it is...excluding the sulfer volcanoes but I don't think those apply

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands