Packing for a day on a small boat is an exercise in bringing enough without filling the boat. Space is limited, everything gets wet eventually, and the sun on open water is considerably harder work than the same sun ashore. The list below is what regulars here actually load, roughly in order of how much they would miss it.
Sun and water
These two are the whole game on the Gulf coast in summer, and both get underestimated. Sun on open water arrives twice, once from above and once reflected off the surface, which is why people who never burn ashore burn badly on a boat. Bring more drinking water than the day appears to require, chilled, and drink it steadily rather than waiting until you feel like it. Dehydration is the most common thing that ends a day early here, well ahead of weather or mechanical trouble.
Clothing
A long-sleeved lightweight shirt keeps more sun off than sunscreen does and is cooler than it sounds once it is damp. A hat with a brim all the way round beats a cap. Polarised sunglasses are genuinely functional rather than cosmetic, because they cut surface glare and let you see the bottom, which in shallow water is a navigation aid. Bring one dry layer in a sealed bag for the ride home, since evening on the water is cooler than the afternoon suggested.
- Water, then more water. A gallon per person on a full summer day is not excessive.
- Sunscreen and a long sleeve. Reapply after swimming, and cover up rather than relying on the bottle.
- A dry bag. Phones, keys, documents and one change of clothes.
- A cooler with ice. Doubles as food storage, drink storage and a seat.
Food and the cooler
Food that survives heat and handling works best: sandwiches, fruit, nuts, anything that does not need cutlery or a flat surface. Ice does double duty keeping drinks cold and keeping any catch in good condition. Pack the cooler so the things you want most often are at the top, because nobody enjoys unpacking a cooler in a rolling boat.
The small things
A phone in a waterproof pouch, a portable battery, a small first-aid kit, insect repellent for dawn and dusk near the marsh, and a rubbish bag so everything you brought leaves with you. If you are fishing, add a towel, pliers and a measuring device, and check the regulations before you need them rather than after. A length of light line is the other thing that earns its space, because there is always something that needs tying down once the wind gets up.
Load it all in one trip if you can. A boat you can move around freely is safer and more pleasant than one packed to the gunwales with things nobody opened.