you built it way too low, you need to build one at least 190 high on y on the middle of the sea, to a mob spawn, it needs light below 7 and be from 24 -> 128 blocks away from the nearest player, so the sea level is 63, 63+128=191, this will prevent from mob to spawning on the shore or in caves, this is important because Minecraft has a mob limit, when this number is reached, no mobs will spawn.

So move your mob trap to the layer 190 and above and in middle of the ocean to properly work, and make sure that is no light entering the mob spawn, if you're in a server, it'll probaly not work nearly as good as sp, because mobs will spawn near other players too.