As discussed above, trim is important. Trim is ever changing on one dive. Most of the elements we cannot control, current, surge, wash and up/down welling. What we can control is what we do in any of the aforementioned.
Here is where I watch what the marine life is doing, and do likewise. After all, it is their environment, so they know best. When there is surge for example, I watch the fish, when they stop in anticipation of a head on surge approaching, I stop. When the surge comes back from behind, the fish swim forward with the surge, so do I. Effortless diving.
Remember also that the air in your cylinder has weight. You have between 6 to 8 pounds of air in your cylinder, depending on cylinder size. As you breathe your cylinder is getting lighter and as your dive progresses you are, as a result, getting lighter. You will become positively buoyant and have to dump small amounts of air to become neutrally buoyant again, trim.