I recently used a sea journey as the starter of a campaign. The first encounter happened in a dockside bar. The characters were loyal crew to a particular captain and got into a scuffle with a rival captain's crew. Before the fight broke out I dropped rumors about a ship limping into port with news of a vast, rich, unexplored land. Rumor spreads fast so the local ruler made a deal with the character's captain, the rival captain, and gave the discovering captain a new ship for an expedition.
The expedition of three ships set out for the unexplored land but were separated by a large storm. The ship that the characters were on was stranded in a sea of sargasso. Much of it was thick enough to walk on and they discovered it crawled with tiny crabs (so there wasn't much chance they would starve). About this time an ooze below deck started attacking the crew. It was later discovered that this was sabotage (presumably by the rival captain) and the ooze had been packed away in a barrel and it had eventually eaten its way through the wood.
A mast in the distance was seen and thinking it might be one of the other ships of the expedition a small group (the pcs) was sent to make contact with it. Turned out to be a wreck, tilted and kept from sinking by the thick mats of sargasso. They start exploring and end up disturbing its occupants: monstrous crabs. Unknowingly they also rile up an undersea race: the squid-men. The squid-men were re-skinned chokers described as humanoid squids.
After the pcs returned to their ship the squid-men began launching attacks against the ship at night. Recurring night attacks whittled down the crew until the captain decided they couldn't stay any longer and sent out a crew in the longboat to row the ship out of the sargasso. It was a long, slow, back-breaking job but they eventually got out far enough to pick up enough breeze to help them escape the squid-men.
Eventually they arrived at the unexplored land to discover that the other ships of the expedition had already arrived. The rival captain had gone rogue and burned the other ship and had managed to incite the locals against the characters ship.
Hope this sparks some ideas for you. Good luck on your adventure!