Getting into a boat from the water is HARD. The drag of the water makes it difficult to jump, and there's often no bottom to stand on. Even if you're in good physical condition, it's quite difficult to heave yourself more than about 15 to 30 cm (6" to 12") vertically out of the water. Take a look at a swimming pool: the copings are rarely more than 15 cm above the surface; in the best modern pools, they're level with it. Most people just can't jump any higher out of the water.
Let's look at Sunset Chaser for a moment:
For the past ten years, the usual way of getting into this boat from the water has been to heel her over until the gunwale is within jumping-aboard distance of the water. You'd then heave yourself up and sideways, a bit like mounting a bare-back horse, and with any luck would land on the seat cushions. This is the time-honoured method of reboarding all small, low-freeboard boats. But you have to be fairly fit, with considerable upper-body strength, otherwise you just get stuck with one leg up on the gunwale. While that can be rather amusing for the rest of the crew, it is somewhat less pleasant for the swimmer.
A far better option is a stern-mounted boarding ladder. Most production boats longer than 5 m / 16' or so have a factory-installed ladder, but they're useful on smaller boats too.
The ladder should angle away from the transom at 10 to 30 degrees. Almost all small powerboats trim stern-down at rest, and it is much easier to climb a sloping ladder than a vertical (or reverse-sloping) one.
50 to 60 cm (about 2') from the first rung to the waterline is usually deep enough, and three steps span this distance nicely. If it's much shallower, swimmers will have to curl into a ball to get their feet on the rungs before pulling themselves up- exactly the sort of awkward physical contortion we're trying to avoid. There's not much point in going much deeper than about 60-80 cm, either. Someone who can't get onto a step 60 cm down will probably have trouble climbing a ladder anyway; if you or your crew fall in this category, you should consider wide, folding steps instead of a ladder.
It's very important that any transom-mounted hardware stays clear of the prop at all times. While you'd never have swimmers in the water while the engine is running, it's quite common for the last swimmer to forget to fold the ladder back up. Ladders aren't sworn enemies of propellers in the same way that rocks are, but don't push it by letting them share the same space. More clearance is better, but a couple of inches will suffice if necessary. Here, we're very tight on space (SC's beam is only 1.5 m) so the ladder-to-prop clearance is acceptable, but a bit closer than I'd like.
When it's not in use, the ladder should be secured so that it doesn't flop around. This doesn't have to be fancy; a light line to a cleat is sufficient.
A good boarding ladder should be rated for about 140 kg / 300 lb. Many are not; the lesser ones often work for a while but are much more prone to fatigue and eventual failure. Higher-rated ladders are usually welded from stainless steel, instead of bolted together with plastic brackets. Simpler is better; you can get a telescoping ladder if you're really short on space, but you'll pay through the nose for a sturdy one.
Markups of 200% or more are not uncommon on good ladders, so hunt around. This one is welded 300-series stainless and cost about $75 (CDN 2011).