There are two things that cause ocean water to become denser, and those are b) the salinity of water and low temperatures.
The more salt there is, the denser the water will be - take the lake Dead Sea in Israel, Jordan, and Palestine for example.
And the lower the temperature, again, the denser the water will be - the deeper you go into an ocean, it will be more difficult to move around due to density and other things.
An organism adapting to its environment leads it to be more ‘equipped’ for survival.
Competition is when other species fight for the same resources that another species needs. Also, it would produce offspring with these adaptations.
An example (to help you better understand);
A blue jay has great eyesight, strong wings, and sharp beak. (Better adapted organism) Another blue jay has great eyesight, weak wings, and dull beak. (The competitor/unadapted)
Both birds find the same food source and are now competing for it. The first bird has sharp beams, which is better for breaking, and ‘chomping’ the seeds/food. The second bird would struggle to consume its food (seed) unless it’s worms.
So which one is better adapted? The first blue jay.
And i hope this helped show how competition can be reduced.