It sounds like you are asking about what gear to take with you on an ocean dive.
Surface signaling equipment are important safety items on ocean dives. It is incredibly hard for a boat captain to see a divers head in the water.
* mirror
* surface signalling sausage
* whistle, and/or Dive Alert, which hooks to your power inflator
Mooring lines often have growth on them that will cut or sting you. Gloves are needed for some dives.
A dive light is useful for looking at things in dark holes and crevices, and can be a lifesaver if you end up lost at sea and are not found before sunset.
You may encounter netting, ropes, or monofilament fishing line. EMT shears will cut fishing line one handed. For a knife, I use a simple folding lockback knife. I have lanyards on both, and the lanyard goes onto my wrist before i use them.
As in fresh water, some sort of slate can be useful. I choose to have a couple of sheets of waterproof paper wrapped around my signal mirror.
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All of the above are things that you take diving.
For on the boat, my recommendation is a small (10 or 15 liter) drybag that you can use to store things like a change of clothing. Having it in a drybag means you can leave it in your gearbag, even if the gearbag may be soaked in seawater during the trip. I find having a small drybag inside my gearbag is a lot more convenient than having to find a dry storage location on the boat.
For a long day out on the water, I also strongly recommend a hat, preferably one that can be soaked in saltwater without damage, and one that has a chin strap to keep it in place in high winds.
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