Transferring gears and sailors between ships while at sea, when they must stay underway as in convoy,
can be quite hazardous. This is done by Bosun's Chair, and there is a great need for steady seas and steady hands for steering.
A dipping can be very serious, dragging the commuters through waves at 10 or 20 knots, so ships can't be too far apart or the line bounces too high just by normal wave motion. If the ships were to close on one another, however, results are even more fatal, so they can't be too close together.
Here's when all hands involved prove their ability.
Photo and annotation courtesy of Jim Reardon