The sea fortress will change from game to game, so you'll need to hunt down hidden escape routes and learn the map's ins and outs each time. You'll be able to scavenge items, smash obstacles, climb around to find new paths or ambush one of the sea monsters looking to make you their latest snack.
For monsters, the task sounds a bit simpler. Hunt and kill all the weak, fleshy humans. So all the time the survivors spend solving puzzles and putting together plans, the monster's getting closer, getting more time to find where they're hiding.
It sounds a bit like nautical-themed Dead by Daylight, which recently added Scream's Ghostface to its list of monsters. If I absolutely must be chased by a horrible sea creature, I'd prefer to do it with a group of pals who I can rely on, and by "rely on" I of course mean "sacrifice so that I can live on and tell the world how brave they were".
Monstrum 2 will be out in 2020.