Transferring gear and people from one boat to another at sea can be extremely dangerous. If there's no other choice, know the safest technique.
The best way to transfer gear and people from boat to boat at sea is to wait until you're back at the dock. Going from boat to boat is inherently life-threatening and a danger to your boats. Being out in the waves means your boat is always in motion, and so is the other boat you're transferring to or from. Worse, since different boats react to waves differently, the boats willbe moving in different ways at the very same time. One rocks while the other rolls, someone mis-times a step, and the potential for disaster is obvious.
That said, there may be times when a boat-to-boat transfer at sea is absolutely necessary. Maybe there's a medical emergency aboard Boat A, and there's a doctor on Boat B. Or someone has a water hose leak on Boat B and there are tools to fix it aboard Boat A. Whatever the case, exhaust all options before resorting to a transfer on open water. But just as you would plan for other emergencies, consider now what you could do should this become necessary.
The safest and most practical way to transfer may depend on the boats involved, weather, sea state, the people involved, equipment, and many other variables. We can't spell out here everything you might need to do, but we can hopefully get you thinking and planning for that event as well as all the other dangerous situations. The following thoughts are generally more applicable to smaller boats.
The first thing to consider is having an experienced (in general and as to the particular boat) helmsperson at the helm of each boat. This is critical and probably that starts with you, now. Know your boat and how she behaves. And make sure the persons involved are wearing properly fitted life jackets.
Getting Or Giving Gear Or Supplies
Obviously, transferring gear is a bit easier than transferring people because you won't have to bring the boats close enough for someone to get from one to the other. Instead, look for ways to get the gear from here to there while keeping the boats as spaced apart as possible. In the case of something that floats or is watertight, like a life jacket or a sealed jug of oil, throw a line from one boat to the other. Then securely tie on the item, drop it overboard, and the receiving boat can haul it in.
When something needs to stay out of the water, it's time to get creative in extending your reach. If one of the boats is a fishing boat, you probably have a landing net handy. The receiving boat crew can stretch the hoop over to the giving boat, who can then drop the item into the net. Or, the giving boat crew can place the item in the net, then hold out the handle for crew on the other boat to grab. A boat hook can be used much the same way, though you may have to lash or otherwise secure the item to its end. In either case, pass the entire net or pole to prevent any fumbling while the boats are in close proximity.
If the circumstances so warrant, always first consider calling the Coast Guard or other appropriate search-and-rescue (SAR) group. These people are usually especially trained and have special equipment for transferring people either from your boat to theirs or to a helicopter for transport to shore.
Moving a person from boat to boat gets very dangerous very quickly because you'll need those boats to be right next to each other. The best way to make it happen safely is usually for both boats to deploy fenders, then parallel each other at minimal speed (but with sufficient power to maintain control). Head the bows directly into the seas or into the wind to minimize rolling motions, depending on which is safest for your location, always considering what is safest for your boat. One boat should be designated the stand-on vessel, which simply holds course. The other can slowly steer closer and closer, closing the gap between them until the boats meet.
If you have capable crew aboard and conditions are good, they may be able to grab a rail and help stabilize the boats. But under no circumstances should anyone place their limbs between the two boats in such a way or place that they could get pinched. As a rule, it's better to let the fenders do their job and hold the boats against each other using the steering wheel. Any additional crew's first priority should be helping to "spot" the person going between the boats and assist them as is safely possible.
In some situations, such as rough seas or when the boats are so different in size that getting to the other isn't possible, even this technique may not be doable. This is a judgment call and requires careful consideration before putting people and/or your boat at risk. If you find yourself in a scenario where it can't be done and you absolutely, positively must get people from one boat to another (such as one of the boats is rapidly sinking) the technique of last resort may be to check out the life jacket the person should already be wearing (as well as everyone else), and tie a line to the person. Use a well secured harness if you have one. If not, a good, well fitted, strong, and properly secured life jacket may do if you have an attachment point that won't compromise the victim's buoyancy and his head-above-water stability.
Pass the line between the boats, and haul the person from one boat to the other with the line. Obviously, this can be extremely dangerous and great care must be taken that the life jacket is properly fitted and completely secure, as is the line. One end of the line should be cleated or tied to the receiving boat in such a way that it can't come free if mishandled, and the other should be secured to the person who will be entering the water, not merely handheld by that person. Careful attention to spinning propellers is crucial. In some instances, it may be best to kill the engines, in other cases the engines may be needed for boat control. But spinning props are extremely dangerous.
If there are waves, consideration of a vessel's downward plunge into the victim is also crucial. Often a victim will want to come aboard at the lowest point, which may be the stern platform. But that could come crashing down on his head. Even the sides of the boat can do this in the wrong circumstances. Transferring people and gear between boats should never be attempted out of anything other than absolute necessity. Unless, of course, you can do it in the safest way possible — back at the dock.