One skill that I have noticed that comes up often is that of cooking.
Good question. What do you want to accomplish?
If you're signing onto a megayacht OR a small boat that is doing the ICW or Bahamas on a casual basis, the focus on technique and recipes is appropriate. I put myself in the talented amateur category in this space.
If you want to be crew on medium (a week) to long (a month) deliveries and passages the game changes. On these kinds of trips the cook is usually the steward and agent. You need to work with the skipper (unless you are the skipper) to understand allergies and medical conditions, including sensitivity to motion sickness. You need to be able to turn a meal plan into a shopping list and provision the boat for 13-15 USD/person/day, including snacks. You need to be able to wedge yourself into the galley and crank out three meals a day no matter how bumpy things are. You need to adjust the meal plan and portion sizes based on sea and crew conditions. You need to be thinking days ahead all the time. Everything changes (again) based on boat outfit. Do you have a reefer? A freezer? How are you doing on ice?
First day out, early departure, lunch of sandwiches (bread won't last long and neither will deli meat). Dinner of roast pork loin and vegetables - make lots.
breakfast - yogurt and bagels (bagels won't last long). Lunch knock out the rest of the sandwiches. Dinner pasta and sauce with salad (lettuce won't last long, hard cook eggs in with the pasta)
breakfast - yogurt and oats with fruit (fruit doesn't last long) lunch - egg salad sandwich (mayo isn't a problem and you can always make it from scratch). Dinner roast or grilled chicken and whatever looks like a salad, including any leftover hard-cooked eggs.
breakfast - scrambled eggs and sausage or soy replacement. lunch - leftover pork sandwiches and cookies dinner grilled fish (if you caught anything) or a stir-fry (what depends on freezer or not).
breakfast - frittata with leftover stir-fry. lunch - chicken salad from leftover chicken dinner - kielbasa and sauerkraut and cole slaw (cabbage lasts longer than lettuce).
There's nothing magic about this plan. It's all about realizing you only have what you leave the dock with and that the more value you can get from each cooking evolution the better for everyone.
On delivery you have to see the boat before you shop because you don't know what you have in terms of infrastructure (reefer, freezer, pots & pans, knives, appliances).
My plan with a stick blender and inverter will be very different from dealing without. No oven? New plan. If I don't have an oven but do have access to a hardware store before pushing off the dock the options increase.
Very long answer to a short question, so I'll return to my first point: what do you want to do?