Just how large of a sea are you considering? The larger the body of water, the more difficult this will be.
How deep does the sea need to be? If the sea is relatively shallow, a hydrothermal field should be able to put out plenty of heat to keep the water above freezing. Shallower waters means less cold water circulating, so geothermal heating could be a significant factor.
Salinity can strongly influence the formation of ice as well, so this could be used.
Imagine a huge underwater caldera on the pole, much of which forms a large hydrothermal vent field (maybe a massive impact crater or perhaps a ring of underwater mountain ridges from plate tectonics depending on how big you want it). The vents produce lots of heat to keep the waters comparatively warm (can be well below the freezing point of fresh water but just above that of a dense brine), as well as release large amounts of salts from the crust to maintain very high salinity.
The ridge around the caldera comes up close to the surface, so there is little significant mixing of deep waters (so the water within the caldera remains a heavy brine with little overspill). The point at which waters freeze is just within the radius of the caldera - when seawater freezes, the salt is forced out of solution in a brine considerably denser than the surrounding water. If this dense brine falls on the inside of the ridge, it maintains and reinforces the salinity of the polar sea.
The combination of hydrothermal upwelling warming the waters (warm is a relative term) and being saturated with salt should prevent the water from freezing at the pole, yet be surrounded by ice just to the outside of the undersea ridges or caldera.