The most useful exercise for dinghy sailors is chin-ups. About the only time you need strength while dinghy sailing is after a capsize, pulling yourself back into the boat, pulling someone else back into the boat, climbing to the up side gunwhale, either on the inside or the outside. And occasionally pulling someone out of the water and onto the dock. (All the other skills needed for dinghy sailing have to do with balance, reaction time, flexibility, and technical skills for things like sail trim.)
Initially, chin-ups may be hard, or even impossible. When I started I could barely do three, and not from fully extended to chin above the bar. Eventually I worked my way up to about 20, though not with very good form. When you start, at first your muscles don’t get bigger – they just rearrange themselves internally to better perform the work demanded of them. If you keep it up, and keep your protein intake adequate, they will get bigger too.
When you do them, keep your palms facing away from you and your hands at about shoulder width (the woman in the picture has her hands just a little too far apart). After all, this is the configuration you’ll employ hauling yourself in over the transom, and not far off from when you’re pulling someone in by the shoulder straps of their PFD. Exercise every other day, since muscle tissue takes about 48 hours to rebuild after the stress of the exercise. Just pull yourself up as far as possible, even if it’s only an inch. No matter how many you can do, keep going until you’ve exhausted yourself to the point that you can barely raise yourself at all. Exercising your biceps to exhaustion strengthens them fastest.
About the only other physical capabilities you need are flexibility and, occasionally, the ability to work to exhaustion. For flexibility, squatting is the most common stress you’ll put on your joints. Bending, twisting, etc. are obviously moves to cultivate. Endurance might come up if you have to fight a capsize in 20+ knot wind and three-foot waves, and if this interests you, look into HIIT (High Intensity Interval Training). But your ability to pull yourself in and up the boat is much more often going to be put to the test.
When you can post-capsize recovery pull yourself over the transom of the boat as the skipper thoughtlessly sails it away, think about a Senior test, if you’re not already there.