2

Two Crabstrositys are fighting for territory, upon noticing the party they stop and attack them instead.

3

A group of merfolk have come together to honor the passing of one of their own- they are mourning in a beautiful tidal pool and have set up a bonfire on shore. If questioned they will explain the deceased was their prince who was slain by a colossal shark that has been plaguing these waters. (It is a CR 10 Giant Shark, give extra HP, multiattack and 1x legendary action)

4

A group of kobold sailors (stranded) have set up a makeshift village. Upon seeing the party they will believe they are rescued and ask to be taken aboard the players' ship. If the players decline saving them they will attack. (Treat as 4 CR 5 Kobold Swarms, Poison Fantic, Normal Kobold, Commando, and Alchemist)

5

A group of Roll 2d6 lizardfolk are having a basilisk race. The party can participate if they want, there is a reward. (Magic item or gold)
In order to participate they take a basilisk that has been blinded and ride it bareback. This will require a series of successful animal handling checks, DC10. Each failed check results in a DC13 Acrobatics/Athletics check to see if the player is bucked off the basilisk. 
In order to win the race, the player must succeed 5 checks in 8 rounds.

6

The party comes across a huge nest of harpies on the cliff-face. If they are noticed the nest will send swarm after swarm of harpies until emptied. 
The nest contains Roll 2d6 swarms of harpies, which are CR 6 swarms with abilities similar to a regular harpy, but scaled up.

7

The party comes across a beached Resolve: {Giant Shark|Whale|Dolphin|Merfolk|Giant Squid|Manta Ray} - they will need to work quickly to save its life...if they want to...

8

The party finds a shoddily built grave, complete with hand-whittled headstone. The sand has eroded and a hand pokes out, revealing a shiny and clearly valuable bracelet. (This bracelet is cursed and was a factor in the deceased's demise) - The Curse is Resolve: {minor|major|deadly}

9

A group of tortle are migrating down the beach, following the schools of fish that are their food-source. They have a few items to trade.They may be able to guide the players to their quarry. Among them are merchants:

Roll on "Merchant Generator" 1d4+1 times

10

A large group of Chuul are excavating the sand in this area. They will attack the party if they come near. If the party defeats them and checks the excavation site they will find an old rusted chest with significant loot inside - including a magic item that the Chuul sensed.

The Loot Includes:
Roll on "Treasure Hoard: Challenge 11-16 (D&D5e)"

11

A small group of Sand Witches are sunbathing, with a picnic of delicious smelling food. They resemble beautiful women but a DC15 Nature/Religion check will determine they are fey. They will try to entice the players to eat the food they have. Doing so will cause the imbiber to suffer 3d6 fire damage and become charmed by the creatures for 1d4 rounds. They may make a DC 13 WIS save to resist this effect at the end of each of their turns. If the witches true nature are revealed they will attack.

12

There is a beautiful golden vessel on the beach. Slightly obscured it takes a DC10 Perception check to notice the glimmer. If cleaned off it will release a water djin who will offer the player who cleaned the vessel one wish (as per the wish spell). The Djin takes the wish very literally, and will attempt to trick the players into freeing it if it can.