Virtually no radioactive fall out from a high altitude burst. The initial release of high velocity neutrons will kill all living creatures within the radius of effect, water is the decelerator of high energy neutrons. This is how the neutrons kill living creatures, the energy is imparted by colliding with the hydrogen atoms in H2O and losing half the energy of the neutrons inside the living creatures every time it strikes another hydrogen atom, it imparts half its velocity/energy. Lots of heat, very bad for living creatures this is how the primary coolant in a reactor vessel removes heat from the reactor vessel and carries it to the steam generator. Underground is also good for shielding and will protect occupants depending on depth. The neutrons will dissipate nearly instantly after the blast, think milliseconds. This is non-ionizing radiation, so radiation poisoning is not a large concern. Being at the bottom of a pool or lake/pond will give sufficient protection depending on the proximity to the blast. By the time you come back up for air, the threat will have passed, except for the shockwave, but you will probably miss that too while underwater. If the blast is too close, you'll boil and get cooked. The alpha's and beta's will be minimal, so long term exposure isn't much of a concern. This type of weapon detonation is for the EMP burst and depopulation while leaving the infrastructure in place for rapid repopulation, theoretically by enemy personnel.