Like schools of robot fish, dozens of undersea robotic undersea vehicles can cruise the near-shore waters. These undersea robots, along with other instruments carried on surface robots, ships, airplanes, satellites, buoys, and drifters, are providing oceanographers with new ways of seeing and hearing the ocean in details. This vision of oceanography is based on a massive robotisation of the sensors... which robot for which application? To answer this question, this presentation first describes different types of marine robots that can be encountered (gliders, buoys, torpedos...). Then a focus is given on two issues in marine robotics: sailing robots which have no thermal and no acoustic signature and then are able to observe the ocean with discretion, and swarm of robots able to collaborate in order to localize themselves without GPS, to transfer real time information despite the low flow due to acoustic communications. A description of various robots available at ENSTA Bretagne is also provided in order to illustrate the experiments. The VAIMOS robot designed in collaboration with IFREMER to perform the analysis of the subsurface of the ocean is presented as the most resulted sailing robot for oceanography