The sahuagins can dig through it
Sargasso mats seem to be so dense that they can't be damaged except by tearing when a
Large creature breaks through. Even a boat will not escape when entrapped.
From my understanding, each 3-dimensional square is treated like difficult terrain where a creature comming from under the heavy mat would have to spend twice (once for 5-foot of thickness each) 2 squares of movement.
It is not specified if it is land or swim speed but it would be fair to use swim-speed when coming from the water to the mat and land-speed when "digging" down in the sargasso.
Mat, light: light sargasso is quite difficult to walk on. Characters must spend 4 squares of movement in order to enter a square of light sargasso mat; running and charging are impossible. The mat is about 5 feet thick, and for 20 feet below that dense tangles of seaweed impede swimmers, who must spend 2 squares of movement to enter a square of light mat.
A Medium creature that ends its move on a square of light mat must succeed on a DC 15 Balance check or break through. A large creature that enters a square of light mat must succeed on a DC 25 Balance check or break through. Huge or larger creatures break through automatically. If a creature breaks through light mat, it creates a patch of open water equal to its space. In addition, climbing out of the water onto the mat again is difficult; a creature trying to climb back on top of a square of light mat must succeed on another Balance check at the same DC, or it fails to climb up out of the water.
Pass: A pass is a stretch of open water in a sargasso. Characters walking atop a sargasso mat must swim to cross passes, while characters swimming through a sargasso find that passes permit them to pass through the seaweed without going around or over it.
Passes are normally 1d4x10 feet wide and meander randomly among the floating mats.
Stealth and Detection in a Sargasso: Characters on top of the sargasso can spot others at a distance of 3d6x20 feet. Underwater, the maximum distance at which a Spot check to detect the nearby presence of others can succeed is 1d8x10 feet due to the heavy cover provided by the seaweed mat.
You could also add patches of light mat or passes to the encounter to allow your sahuagins to hide in clean water waiting for your players to be in reach.
(I do not know where you've taken this terrain block from. I used this sites that looks similar : http://www.realmshelps.net/adventuring/aquatic.shtml)