On a stormy night, a ship out of control with torn sails is thrown against the jagged rocks close to shore.
The players might be on the ship themselves for whatever reason or might have seen the ship slowly get closer and closer to shore and witnessed their struggles against the wind and waves.
The ship itself strands some distance from shore and a large gash is torn in the side of the ship, flooding its hulls, leaving it to sink fairly quickly after. The passengers who survive the impact and not being washed overboard by the crashing waves can make their way onto the the rocky formation they crashed against.
If the players are on shore they see a jetty with a long oar boat attached to its end which they can use to row out against the waves to try to save some of the crew and passengers. When about half way there, they start seeing these humanoid shapes crawling out of the sea onto the other side of the rock to the survivors. These creatures seem somewhat off as their forms glisten in the cool moonlight when it washes over the rock momentarily in between the stormy clouds.
These creatures are sea spawn (as found on page 189 of the DnD 5th Ed Volo’s Guide to Monsters), formerly human but transformed into a grotesque mix between their original form and various creatures of the sea. The sea spawn will try to kill, feed on and drag survivors into the cold depths below, and not necessarily in that order.
The players are faced with a decision here whilst rowing out, do they save their own skins and turn back or continue onward towards the rocky outcropping amidst the breaking waves to save the survivors of the shipwreck.
On the other hand if some or all of the players were on the ship, they have to fight for theirs and others lives, and try to survive long enough to get onto a long oar boat coming from shore.
The battle can all take place on the rocks or extend into the waters below, where these creatures might have a shamanistic leader who commands sharks or other sea creatures to make the encounter more difficult, or the sea spawn might continue harassing the people as they try to row to safety. They might try to pull the boat over to the side to flip it over and so forth.
After this encounter you can extend this into an adventure arch if you’d like. The players might encounter a water elemental creature as they return to the wreckage the day after to look for other survivors or valuables in the wreckage. This creature would be curiously siphoning through various humanoid trinkets as they interest its curiosity.
The being will communicate with the group that it needs the players aid in removing these sea spawn from its local waters. The water elemental being has magical means of allowing the group to breathe underwater for a certain period of time.
The sea spawn are located quite a bit further out, in a deep chasm, where they bring offerings and pray to their dark gods of the deep.