【Eddy】 , 【whirlpool】 , 【maelstrom】 , 【vortex】 mean a swirling mass especially of water.

【Eddy】 implies swift circular movement (as in water, wind, dust, or mist) caused by a countercurrent or, more often, by something that obstructs; it is usually thought of less as dangerous than as annoying or confusing.

【Whirlpool】 suggests a more extensive and more violent 【eddy】 in water; usually it implies a force of swirling water (as at a meeting of countercurrents) so great as to send whatever enters whirling toward a center where it is sucked down, but it may also be extended to other things that draw or suck one in like a swirl of raging water.

【Maelstrom】 is basically the name of a very powerful 【whirlpool】  off the west coast of Norway which was supposed to suck in all vessels that passed within a wide radius; the term is extended to any great turmoil that resistlessly drags men into it.

【Vortex】 usually suggests a mass of liquid (as water) or gas (as air) rapidly circulating around a hollow center; it is visualized principally as something which draws all that become involved in the swirling into its center.