I was told that the total integral of the stress over the surface of a swimmer (i.e. the total force exerted by the swimmer on the fluid) always vanishes, because there are no external forces applied on it. That seems fair by the 3rd Newton Law.
But, how does it take into account the effects of the 2nd Newton's Law? If, for example the swimmer starts from a stationary state, and at the time $t_1$ reaches a velocity $v_1$, where does the force generating the acceleration $v_1/t_1$ come from?
I was told that this can happen just because in the case of a swimmer with density different than that of the fluid, so gravitational forces are present. But it seems bulls**t to me. Could you help me to clarify all this?