Mapping monster territories not only helps with encounter tables, it also allows the DM to consider expansion and replacement of species* as well as new hybrid monsters when two formerly separated creatures finally come into contact.
Some marine monsters are restricted to specific ocean currents. These are usually archmage or divine imposed limitations.
One of the most handy devices for defense of a ship that I have never seen mentioned is the flying broom. A few of them allow for easy scouting as well as attacks from above on mers or sahuagin trying to take the ship. Enough could even pull a smaller vessel when the wind has died down.
Some swarms have traits and powers not found in the individual creatures. Inspired by this weird report on worms: https://www.sciencenews.org/article/how-wor...iquid-and-solid
* This may sound boring but the character will definitely have something to worry about when the local orc tribe is exterminated and replaced by gibberlings or trolls.