Ok, I'm thinking about a seagoing campaign and want some nice curveball ideas to throw my players ways whilst out and about on the ocean and I had one idea for a shipwreck mimic. Anybody else want to contribute reflavors of current monsters to the pool? Not really looking to homebrew, but some revamped normal enemies would be nice. Krakens, island/dragon turtles, skeletal, ghostly and run of the mill pirates are all obvious shoo ins. But what else do we got swimming around in our collective brain buckets?

Should you want non-combat encounters and ideas to put your players in a jam as a break, here's some ideas:

• Plague ships passing by (a risk-reward for any loot that's left)
• Umberlee's storms that might be appeased by throwing out a valuable treasure
• A serial cheater at knucklebones that's riling up the crew
• A game to while away the time, whether or not it has been approved by the commanding officers aboard.
• A haul of fish that upon closer inspection could infect the crew
• Water purifying equipment needing repair, so to save the resident spellcaster some slots when creating drinkable water.

I hope that helps, as I find non-combat encounters to be refreshing while keeping players on their toes for the moment.

Currently running a seafaring campaign and have some fun ideas to throw at them:

• Weather conditions (either no one causing a delay or a huge storm) mean the ship is running lower on food then they  expected and won't have enough to make the full journey as expected
• A murder occurs on board so now there is a killer on the ship and the party has to find out who and what to do (in mine the killer was actually influenced by an unknown fey prisoner on board the ship)
• While a kraken/giant sea monster is a pretty common trope, it is pretty cool to fight off a massive monster and have your party blasting and slashing something
• Someone on board has became a drug dealer. This will be a moral quandary for some LG type players, particularly if the drug may have negative side effexts
• A wizard pirate has created an airship and is terrorizing/raiding a lot of ships and has to be stopped. Also provides some good magic loot if you are so inclined
• I created a floating city that travels the seas but is self sustaining. Could have unique items or an escalating conflict. Lots of different options with what to put on based on your overall campaign

Here is a nasty, nasty seafaring surprise.

Your ship has made a habit of raiding innocent island folk/merfolk.  They routinely kill and steal from a bunch of helpless fishermen, every trip they make.  And they just came to the island.  

Will the party stop the sailors?  will they participate?   Will they look the other way?

This can change the trajectory of the adventurers, particularly if they have some goodie too shoes but no one knows how to sale a boat. 

• The elder elementals as environmental hazards/ living storms are cool.  [Tooltip Not Found] 
• Sirens are fun, there is no good monster but choosing something like a satyr reveler and giving it a swim speed or sea hag. A fun reference could be a sea hag with an eyes of charming which is an amulet containing a mermaids voice. 
• oni pirate after magic items 
• an ancient dragon turtle island 
• A sea dragon. They could be in a fight and need help, they could be dredging up an old ship and ask the party to help clear it out, they might ask for tribute. 
• I like the plague ship but you can have a variety of  ships including zombie infected, traders, pirates, stranded crews, cursed or illusions