In this specific scenario, not only is it not a crime, failure to do so is a breach of proper maritime protocols and could be illegal.
A person manning a boat and sees a human in water in distress should immediately go to the rescue or recovery of the victim. This is commonly started by the crewman of the vessel who spots the body shouting "Man overboard" once... which triggers every crewman on the deck to also shout the same thing exactly once regardless of if they can see the man who is overboard (The idea is that the call is so all crew on the deck are not announcing the sighting but responding that they know there's an emergency situation in progress... and to muster to the appropriate situations). The boat's helmsman or pilot will then proceed to maneuver to recover the body in the water and bring it onboard the boat, (if traveling by sail, to then as soon as possible transfer to a motor powered vessel or to land which ever is quicker).
Additionally, should the boat have radio communication, the situation should trigger a mayday communication which will alert other vessels, regardless of purpose and local emergency services to the situation, with GPS tracking aiding all responders. Typically while underway at sea or in large bodies of water, all operators of watercraft are required to respond, regardless of actual purpose on the water (recall the famous "Miracle on the Hudson" incident where New York Ferry Operators were the actual first responders to get ALL passengers and crew off the crashed plane to land, where EMS and police services were waiting to deal with the survivors). Most boating licenses these days require a basic understanding of duties with maritime safety and if you come upon a situation while underway, you are a first responder automatically.
Remove the question of water and say that there is person who you are unsure of if they are alive or dead on dry land. Under law, you cannot be held liable for anything you do in the course of saving a life so long as you are acting within your competency to do so, and do not take actions you are not licensed to do as part of training, oversight, and regulations dictate, you cannot be sued for medical injuries inflicted on someone in the course of saving their life (called "Life over Limb"... that if the only way to save an unconscious person from death is to amputate a limb, you're not liable for cutting off the limb in question, so long as you are trained to do this. To a lesser extreme example, if CPR is performed right, you will break the victims ribs... since it's a question of keeping the person alive, the temporary pain and handicap of broken ribs is seen as acceptable).
As a lifeguard (for pools, I wasn't certified for "wilderness lifeguarding" but the differences in this respect were minimum) your first thing to do when getting in the water to fish out a swimmer in distress was to get them back to the pool deck before life efforts could occur (there are a few times where you had to start the life saving efforts in water, but I can count them on one hand, and one of those incidents was only after confirming your victim was conscious and even then you still were moving to get them out of the water asap... it's just asap would be a little bit longer. We also weren't trained to pronounce death. If it was so serious that CPR starts, you do not stop unless another guard or EMS relieve you.).
In these situations, there is proper evidence gathering procedures that will be used to establish what happened. For example, the first responders will be fingerprinted and prints in the areas touched for life saving efforts will be noted as part of the evidence. Additionally, paramedics, EMS and first responders will often alert police to any items of note they saw around the victim that were disturbed in the process of life saving efforts. The police may collect evidence from you to rule you out as a suspect (by showing the evidence on the body was consistent with applied life saving techniques) and to identify evidence not related to you at all (a pink fiber on the body that doesn't match any of your clothes means the body came into contact with the fibers from another source).
This also establishes "Chain of Custody" when you call the body in and stay with it and allow minimum contact with it (many larger ships will store the body in a locked room to keep crew and passenger contact to a minimum). Either way, so long as you tell the police everything you did in rescue and recovery, this isn't tampering with evidence on the face of it. If something that is inconsistent with your story comes up, it's not necessarily incriminating either... in the heat of the moment in a rescue and recovery effort, tunnel vision can form and you aren't expect to recall everything you did. Police should know this and understand that you were cooperating in what for most of the world is an unusual situation.
