There may come a time when you must slip over the side to remove and replace the propeller. While inconvenient, so long as the water is clean, the procedure doesn't pose much of a challenge. That is, unless a tool or the propeller itself slips through your fumble-fingers to sink in 60 fathoms. Such a dilemma can be prevented, though, by having the right tools at hand.
It's common practice to remove the propeller cotter pin with a pair of Channel-lock (slip-lock) pliers. Notice how the adjustment slot leaves ample room to attach a small clip like the style found on a dog leash. Spring-loaded clips are readily available at hardware stores. Attach a thin cord to the clip. Some 1/4-inch nylon parachute cord from an army surplus store works great. Loop the bitter end of the cord around your neck, or optimally, tie it off on a cleat. This way, you not only don't have to worry about dropping the pliers, you can intentionally let go of them when it's time to grab the propeller nut. Then, should you need them again a minute later, retrieval only takes a second.
Most Crescent wrenches have a hole conveniently located in the end of the handle. For those wrenches, pliers and screwdrivers that don't have a hole, add one. Epoxy a washer to the handle where its positioning won't interfere with the use of the tool. Be sure to rough up the mating surfaces to insure a physical bond with the resin. If you have access to a welder, theoretically you could weld a washer to the shank. However, it won't look as nice and the fumes created by welding could be toxic (cadmium, chrome). It's better to keep things simple.
To keep from dropping the prop nut, use one of those mesh wallet bags that hang around your neck. Slide it over the loosened prop nut and unscrew it the rest of the way off the shaft. Use the mesh bag the same way you'd use a dish towel to open a really tight juice can lid. When the nut comes off, it drops neatly into the mesh bag. The mesh bag is either looped around your neck or anchored to a cleat. Of the two methods of anchoring, tethering to a cleat works better. A cord wrapped around your neck can become a garrote. As you unwind the nut, the cord winds tighter and tighter.
Next do the propeller. Loop several wraps of parachute cord around the base of one of the blades, then knot it tightly. Don't worry about being able to untie the knot. The cord costs less than a propeller. Do a second blade the same way. Because of the weight, the bitter end is secured on deck.
If this seems a clumsy method, there is an alternative. Drop a tethered mesh bag overboard. Loop it so it falls forward over your shoulder and in good position to bag the propeller once the prop nut is removed and the wheel slides loosely on its shaft. Bag the prop while its weight is still supported on the shaft, just like you did with the prop nut. Be sure the mesh bag is strong enough to hold the prop without breaking through and plunging to the bottom of the sea.
Another method entirely is to construct a floating tool caddy. One popular design relies on an inflated inner tube with a mesh basket secured by wire ties to the inside of the "doughnut hole." This basket should not be too deep, otherwise it's too hard to retrieve tools. Remember, you're in the water reaching over the top of the tube and down into the well. Conversely, neither should the net be too shallow. Otherwise wave motion might pitch the contents into the sea. The best set-up would be a combination of tethered tools used in conjunction with a floating caddy.