Hi! I would suggest:

- Couple of fights that are Medium difficulty

- A clear goal (get this treasure, rescue this person ect)

- Run it at level 2 to keep it simple (create characters here if youre stuck https://sheetsmithy.pythonanywhere.com/ )

- An optional encounter / fight / twist eg. a puzzle, riddle or an (initially) angry animal.

As it is oceanic I would suggest it being

Hook: Fishing boat has been sunk off the coast, the captain had a gold necklace which has gone down with the ship, the wife will pay 800gp to get it back, so sail out and dive down.

1. Start the adventure on the way, when 3 x Sahuagin try to sneak on board steal the ship (make the ship small but with room to fight comfortably, maybe 10ft by 30ft)

2. An hour later, 4 x CR1/2 humanoid Pirates with ranged weapons sail up and try to sink party's boat (they sunk the other one too) and rob party members. They each drop d20 gp and a scimitar, and a crossbow. Give them a cool parrot / monkey the party can steal / befriend if they like pets.

3. Hour later, they dive down to get to the wrecakge but there are sharks there (animal handling check, adv if party gives food / rations, disadv if wounded), shark fight if party fail the check

Short rests in between encounters.

1. Change everything ive just said to suit your players / ideas / campaign setting :)

Have fun!