Another thing to consider in addition to what temp the ocean is would be how specific gravity, salinity and temperature interact. Specific gravity indicates density, while salinity refers to the actual weight of the salt. As the temperature of the water fluctuates so does the density of the water. If you want to see exactly what I am talking about, there is a chart that details it on www.reef-aquarium.net
on the educational resources page...Left side, a little ways down. Click on "Salt water and specific Gravity."