While pure water freezes at 0 degrees C (32 F), salt water freezes at -1.9 degrees C (28.4 F). But salt water doesn't freeze consistently either...it tends to have large areas of mostly pure water and then it traps veins of briny saltier water that may not freeze until temperatures as low as -21.1 C.
So that is an important factor and I understand that the lower temperatures of the ocean water observed in the Pacific and Atlantic Oceans are actually caused by the polar ice caps melting. I'll come back to this point.
Now the polar ice is melting mostly because of the warmer surface temperature...it is predominately the ice above sea level that is melting. Sea water is basically a suspension of salt (and other particles) in water. But when water freezes, it has a crystalline structure that tends to push out impurities. So sea ice is large chunks of fresh water with veins of brine and "dirty" ice that freezes last at a much lower temperature. And that dirty ice may not even be entirely frozen. Water's unique property of being less dense in frozen form than it is in liquid form factors in here, because the more pure ice floats to the surface while the dirty ice, being more dense, tends to be at the bottom under water unless insulated by clean ice. Surface temperature rises, fresh water is melting off the surface and the insulation for the really cold dirty water dissipates and it also sloughs off into the sea.
So, temperature of the ocean: It has been observed in both the Pacific and the Atlantic Oceans that the water temperature has lowered. The cause is the polar ice melting. The oceans collectively have a big wobbly loop cycling through all of the oceans pulling colder water from the poles, cycling through the warmer Atlantic, Pacific and Indian oceans and pulling that warmer water back to the poles. Or at least that's what it's supposed to do. Instead, it's pulling COLDER water from the poles, depositing it in the Atlantic and Pacific. To make matters worse...that current is supposed to result in the water warming in the tropics before its being sent back to the poles. But that water is starting colder, doesn't have enough of a chance to warm up and it's going back to the poles colder than it did decades ago.
Now here's where things get fuzzy.
Climatologists have hypothesized that the ocean temperature has an impact on our weather. With a lower water temperature and a higher surface temperature, this can create some turbulent high and low pressure areas, particularly near the coastlines. The weather will grow more erratic and more dramatic, eroding the coast and impacting micro-climates up and down the seaboard. This is a much larger concern than ocean water levels rising. But I want to reiterate: It is a hypothesis. The data has not yet been conclusive. Scientists continue to collect data all over the globe and analyze as best as they can. There is fairly clear indication that something is happening...but the pieces haven't been put together completely. It's kinda like playing connect-the-dots. When you're going from dot to dot, you probably don't have to complete the sketch before you have figured out what it is. We may not need all of that data before we realize that we have a problem.
Side note, because this came up in a debate earlier in the month: Weather is the daily occurrences on a regional scale and it can be observed by walking outside. Climate is the trend on an overall global scale of the collective weather and it cannot be observed without collecting data across several years. There is a significant difference. Neil Degrass Tyson explained it pretty well using the model of a guy walking his dog on a leash along the beach. The dog would represent weather and he may wander wherever he wants within the confines of the leash (and he will, if you know dogs). But the Climate is the man at the other end of the leash and he has a more linear path.